My new 4 yr old ottb is a bold jumper, but he doesn't respect showjumps. I've tried fillers, etc. but he still tends to trash them. Any ideas to get him to be more careful?

If he is hitting the poles try getting some heavy wood planks so that when he hits the pole it hurts more so he'll want to pick his feet up better and be more careful, you can also fill regular poles with sand and block off the ends so it doesn't fall, I've also seen people put metal planks over the rail so that if it gets hit it makes a sound
Grids are great, like said before ^^ they'll make him think about where he's puttin his legs. Xc is good to because the jumps are solid and they don't want to knock it, being young he's a bit green so it's something that will come in time with more work, good luck! :)
Grids - make him think about where he's putting his legs. They'll also help him get better shape over jumps.
Some small cross country jumps, not to big at first as he may hurt himself x
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