The thing that helps flanking is width. The map needs to be wide enough to allow for flanking but not so wide that blobbing is OP. For example playing Steppes 3v3 is just a blob fest, zero fun.
I do think one factor here besides pure width is what the middle of the map is made of. Steppes is inviting to blobbing because the entire middle of the map is just a wide open area with very limited cover and sight blockers. Add a few buildings, hills or forests in there and it should become much better. The open plains offer no ways to hide your approach and all that is left is to go on a full-on assault (aka blob) and hope for the best.
This is what makes General Mud much better to play (less blobbing and less arty), despite being pretty much as wide (as Steppes), because the entire middle of the map is buildings, small walls and negative cover roads. But it's still open enough to allow for lots of flanking.
I really think General Mud is pretty much the best 4v4 map there is. La Gleize is a close second, if only it had the munitions locations a bit more spread to the sides. Lazereth would be a solid third if it had the bases a bit closer together and if it had a better distribution of the middle sectors (being on the flanks is quite boring). Although I always have some of the most epic games on Hill 400 for some reason.