I've seen both Ebm and Edim7, probably about equally. It depends on what line you want through the chords. Edim7 gives you a rising bass line in bars 5-6: Bb-D-Eb-E (and then to F on the next Bb). Ebm gives a descending line, usually in a higher voice: Bb-Ab-G-Gb (leading to F from the other direction of course).
Ab7 is rarer, and would be a sub for Ebm. (bVII is quite a common sub for a minor iv, but I've not seen it much in rhythm changes. Sounds good, mind.)