stuck on flpe 4

I've been on flpe 4 step 9 for months. I just can't seem to get past the last set with decent form. From the start sets 1-3 have been fairly easy. Recently I've also more frequently managed to complete set 4 with acceptable form but latest by set 5 rep 40 my form is down the drain. 
I've kept at it for months on end, but can't seem to break through. One major detail I stumbled upon a couple of months ago in the forum is trying to keep your belly from bulging, actively pulling the abs in and down. That made it a lot harder esp. harder to breathe. By set 4 I'm gassing due to the pressed breathe, but on the other hand it helps a lot to maintain good form and keep the hips/legs from flopping around. 
Over the course of just a few weeks I noticed a significant improvement in hb form and compression strength. flpe set 1-3 are so easy that I sometimes raise my hands overhead because I like the position better and it helps the rocking.
Nonetheless, I keep breaking in at set 5 rep 40 no matter the hand position. I can't keep the abs pulled in and therefore can't keep a tight core with zero hip/leg movement.

Yesterday I tried 6x48r to build endurance and will stick with it for a while to see if it helps. Any other suggestions? Any tips regarding diaphragm breathing?? 

Hey Bas how much proficient are you on the front split?

10 hours ago, Alessandro Mainente said:

Hey Bas how much proficient are you on the front split?

I don't have the front split series (yet), so I'd say not proficient at all.
I do the middle splits but haven't stuck with it lately.  Why are you asking?
 

I'm asking that because on PE4 if you want to hold the PPT you have to open the legs in a straddle position no more than 90°, at this point the leverage on the ileo-psoas and iliacus it is higher and if they are tight they tend to win against the abs and you begin to arch the lower back or , if you want to see from another pov, the abs begin to suffer and lose strength and especially endurance. I solved these type of stall point with a deep psoas release with front split work.

8 hours ago, Alessandro Mainente said:

you want to hold the PPT you have to open the legs in a straddle position no more than 90°, at this point the leverage on the ileo-psoas and iliacus it is higher and if they are tight they tend to win against the abs 

Alright, gotcha! During this exercise I sometimes get cramping in the tfl area and my piriformis is notoriously tight, which I feel is part of the problem. Will do some stretching and release. I was actually going to get the thoracic bridge stretch next...but now you got me thinking. I never liked excessive stretching esp not the legs and hips. Guess I need a lot more of it :/