TOSPHOS ON DAPH 7 AMUD 2 SAYS THAT AVRAHAM AVINU WAS 73 YEARS OLD WHEN HE FOUGHT AGAINST THE 4 KINGS.
IF HE LEFT ARAM (THE SECOND TIME) WHEN HE WAS 75 AND THEN WENT DOWN TO EGYPT WITH LOT AND AFTERWARDS LOT MOVES TO SODOM , HOW COULD THE WAR HAVE BEEN WHEN AVRAHAM WAS 73 YEARS OLD.
Below is what we wrote regarding a similar question that was asked on our forum.
Kollel Iyun Hadaf
Berachos 007b: The age of Avraham at Bris Bein ha'Besarim
Kobi Schlesinger asked:
Question:Tosfot explains that Brit Bein Habetarim happened when Avraham was 70 years old. and the Kings War happened just before it.
Its very hard to understand how it can be, because it said in the torah that Avraham was 75 when he got out of Charan. and that was before Lot sattled in Sdom, and Avraham lived in Alonei Mamreh... --------------------------------
The Kollel replies:
The intent of Tosfos is to explain that the words, "v'He'emin ba'Hashem" -- "And he trusted in Hash-m," are the end of the Parshah of the prophecy of Avraham which he prophecized after the War of the Kings. The verse, "ba'Meh Eida," is the beginning of a new Parshah which returns to the prophecy of Avraham at the Bris Bein ha'Besarim, which occurred five years prior to the War of Kings (that is, it is a flashback; "Ein Mukdam u'Me'uchar ba'Torah" -- the Torah is not strict to keep to chronological order).
As far as the reason why the Torah placed the prophecy of the Bris Bein ha'Besarim right next to the prophecy that occurred after the War of the Kings, see the CHIZKUNI (Bereishis 15:7) who explains this at length.