My slime block sweeper nether mob farm was working normally up through 1.13-pre3, but with pre-4 it seems the timing of the redstone signal has changed, causing the traveling pistons to no longer sweep back and forth. Instead, they just jitter in place. Normally a clock sends a single tick pulse which becomes two quick pulses in succession, driving the piston pair to travel along a rail. Now the pistons act as if it is one single longer pulse and just stay in place.

It uses a clock with two observers and two repeaters - one on 1 tick, one on 2 ticks to drive the signal.

EDIT: It definitely has to do with the observer clock. I made some changes, and now the sweepers work. I attached two pictures of the changes I had to make.