Yeah, the collapse scrum button is Square, I believe. It's in the controls instruction in the Options portion of the PS4 version. Here's something I posted earlier on the RUGBY 15 forums. Putting it here, because here is better.
Here are my tips for getting the most out of Rugby 15.
Use a Bad Team
The game is too easy. The CPU won't ruck. So use a bad team to make up for it.
Use The Tactics
If you want to pick and drive, go to Pick & drive. If you want to crash the ball, go to Crash Ball. If you want to fling it around, go to Expansive. If you want to kick, well, just box kick the thing and you'll be fine. As an added tip, I'm finding the Crash Ball option makes for good, creative rugby, with the odd switch and players hitting their gaps hard.
There is plenty of time in the ruck and during set pieces to change up your tactics. You can literally do it with every phase if you want to.
Pass It Fast
If you run more than a few steps with the ball, the backline will start to run support lines. This is part of the unpredictability factor of the backline. If you want to hit players in their channels, pass it quickly.
Kick More
Kicking, especially box kicking, is nice in this game. You can use the wind to make some really savage kicks that are quite satisfying.
That's it. Those are the things that are making it more fun for me. I'm going to do an experiment tonight and turn the rucking ratings WAY DOWN on Ospreys to see if that makes the game more fun.