I think shorter days will be easier on all of you, but especially the babies. I would vote for 4 5-hour days. If it is totally flexible, I would work M & T, have W off, and work Th & F.
I went back to work when my dd was 13 months. I always came home for lunch, so the longest we were apart was about 5 hours. I worked 32 hours per week, as 4 8-hour days. And I took Wednesdays off. This part worked great for me. I used to say, "just when I get tired of coming to work...I get a day off!"
Hope it works out well for you and the babes.
ETA: After DD turned 3, I started working full days. She was still nursing, but only first thing in the morning and at night. She didn't seem to need me to come home at lunch any more. I experimented for a while with doing 5 day weeks, ~6.5 hour days. This trade-off did not work for me - I needed that extra day off. Now my DD is almost 5, and the extra day off is Friday.
When I lived in Europe, all the working families - mothers AND fathers - worked 32 hour weeks. I think it is the only sane thing to do!! I work just enough to keep health insurance for the family. At my current position, that is 30 hours per week. Yeah, I could earn extra money by going up to 40 hours, but I would be even more stressed, tired and overall not a happy mama. My two cents.