I haven't personally tried the fourth element ones. I have helped a few guys don them on the boat, they seem hard to seat.
It is just a generally poor design....the sealing o-ring has a tendency to push out of its groove when pressing the two halves together. To address this issue, Fourth Element made a running change to the rings where the groove jumped from approx 5.5mm wide to 6.xmm wide....this was supposed to allow the o-ring to move (roll?) in its groove instead of being pushed out. While the change was somewhat effective, it did not completely eliminate the problem nor did it make the two halves of the system much easier to align and press together.
-Z