Zipporah with Moshe – According to this approach, Zipporah and sons were living with Moshe in the Sinai Wilderness while he shepherded Yitro's sheep,2 and the verse in 4:20 records how "Moshe took his wife and his sons and set them on a donkey (to send them back to Yitro, while/before) he (Moshe alone) returned to the land of Egypt." As opposed to all other exegetes who must grapple with the absence of any verse which explicitly recounts Zipporah's return to Midyan, R. Chananel and Seforno say that this is the very trek described in the first half of 4:20.3
Permission from Yitro – Before Moshe sends Zipporah and their children back to her father, he first needs to obtain Yitro's approval for his plan, and thus Moshe returns to Midyan in 4:18. While Moshe was in Midyan, Hashem tells him (4:19) that the timing is now right for his trip to Egypt.
Split screen and who was at the inn – R. Chananel maintains (like R. Saadia below) that Moshe sent Zipporah and their sons to Midyan by themselves and was not with them at the inn – see Mystery at the Malon for the motivations and ramifications of this position. According to R. Chananel, after Moshe and Zipporah go in separate directions in 4:20, the Torah tells us about what happened to each of them on their journeys home. First, 4:21-23 records Hashem's communication with Moshe on his way back to Egypt, and then 4:24-26 describes the concurrent events at the inn which occurred to Zipporah on her way back to Midyan. In contrast, Seforno asserts that Moshe accompanied Zipporah and their sons to Midyan and was present for the incident at the inn in 4:24-26.4
On the Way to Egypt
This approach subdivides regarding the point during the journey at which Zipporah was sent back:
Before the incident at the inn
Initially, Zipporah and the children were accompanying Moshe back to Egypt, but Moshe changed his mind on the way and sent them back to Yitro before the incident at the inn (4:24-26).
Moshe's change of heart in the middle of 4:20 – According to R. Saadia, 4:20 should be interpreted as follows: "Moshe took his wife and his sons and set them on a donkey (to take them to Egypt, but he then reconsidered and sent them back to Midyan) and he (Moshe alone) returned to the land of Egypt." This reading is particularly difficult, as there is no textual indication or even subtle hint of either Moshe's reconsideration or Zipporah's return to Midyan. Additionally, it is unclear what would have motivated Moshe to change his mind.6
Split screen and who was at the inn – R. Saadia explains that Moshe sent Zipporah and their sons to Midyan by themselves and was not with them when the incident at the inn took place – see Mystery at the Malon for the motivations and ramifications of this position. According to this, 4:21-23 records Hashem's instructions to Moshe on his way home to Egypt, while 4:24-26 describes the simultaneous events at the inn which befell Zipporah on her way home to Midyan.
After the incident at the inn
Moshe sent Zipporah and their children back to Yitro's home immediately following the incident at the inn in Shemot 4.
Moshe's error – Ibn Ezra and Tzeror HaMor both criticize Moshe's decision to bring his family with him, but for different reasons. Tzeror HaMor views it as procrastination,8 while Ibn Ezra suggests that it was a tactical mistake as it would send a message to the people that he is coming with his family to settle there and that the Exodus would not be imminent.9
Incident at the inn – While Tzeror HaMor sees the event as a punishment for Moshe's stalling, Ibn Ezra views it as a means of preventing him from taking Zipporah and sons to Egypt and getting Moshe to realize and correct his error.10 Ibn Ezra does not explain why Hashem didn't simply appear to Moshe and tell him that it would be better if Zipporah stayed in Midyan. He also doesn't address why the Torah doesn't explicitly say that Zipporah went back home after the close call at the inn.
Chronological order – According to Ibn Ezra "and he returned to the land of Egypt" in 4:20 refers to Moshe alone (without Zipporah and their children) and this happened only after the story at the inn recorded a few verses later.11 Ibn Ezra also posits that 4:21-23 are achronological and occurred before Moshe set out for Egypt.
After meeting Aharon
Moshe sent Zipporah home only after Aharon met him and suggested that there was no point in bringing more people to be enslaved in Egypt.
Gap in the text – According to the Mekhilta and Rashi, not only is Zipporah's departure missing from the text, but there is also no hint of such an exchange between Moshe and Aharon.
Moshe's change of heart – the Mekhilta and Rashi do not explain what Moshe was initially thinking, and why he was persuaded by Aharon's argument.
Returning Zipporah – The Akeidat Yitzchak suggests that Moshe behaved wrongly by not sending for Zipporah and his children immediately after the Exodus, when there was no longer any reason that they could not join him.
After Arriving in Egypt
Zipporah returned with Moshe to Egypt and went back to Midyan only afterwards.
Change in plans – Ramban also thinks that Moshe's plans changed, however, his suggestion goes in the opposite direction of the Mekhilta, R. Saadia, Rashi, and Ibn Ezra above. Ramban explains that originally Moshe planned on returning to Egypt by himself in disguise, and this is what he told Yitro in 4:18. However, when Hashem informed him in 4:19 that his enemies had died, Moshe could now go back openly and also take his family with him.13
Family accompanying – Shemot Rabbah suggests that this is important for personal reasons, so that they can participate in the Exodus and revelation at Sinai.14 Ramban and Ibn Kaspi also view this as a positive development, but as being for the benefit of the nation that they should be imbued with confidence that the Exodus will be happening soon thereafter.15
Zipporah ultimately departed – According to R. Yosef Bekhor Shor, Yitro went down to Egypt to pick up Zipporah and take her back to Midyan. According to Shemot Rabbah and Ramban, after his initial failure to persuade Paroh to let the people go, Moshe himself went back to Midyan with Zipporah for six months.16
When was Eliezer born? While Eliezer's name is first mentioned only in Shemot 18:4,17 the plural form of "בָּנָיו" in Shemot 4:20 would appear to indicate that he was born before Moshe returned from Midyan to Egypt.18 Ramban, however, argues that the plural form is not conclusive,19 and he suggests that Zipporah may have become pregnant with Eliezer either on the way to Egypt or after they arrived in Egypt.20