I want to use a BMX style handlebar on my MTB. The BMX had a 4-bolt handlebar binder set. The MTB has only a clamp-on 25.4 ahead stem with 2-bolt handlebar binder set. The 2-bolt binder set won't clamp firmly enough to keep the BMX handlebars from rotating inside the binder clamp. Is there a clamp-on stem set with a 4-bolt handlebar binder set? Otherwise, any suggestions on how I might be able to keep the BMX handlebars from rotating inside the MTB's 2-bolt handlebar binder?

Your problem isn't with the number of bolts, but with different diameters. BMX bars usually are 22.2 mm while MTB bars usually are 25.4(with oversize bars available at 31.8 mm too). Get a pair of 7/8-to 1 adapter shims and you're all set.
Do note that BMX bars, being higher, will put more stress on the stem than the original bar would. In the interest of safety you might want to get a strong DH stem to use.

you can get BMX bars in the 25.4mm diameter.
but you cant get mountain bike gear shifters to fit that size.

you could use a BMX aheadset stem. But they have not much extention on them.
still that would mean you would have bars nearer to you

but might mean your knee hits the bars. Especially when starting from a junction.

I had BMX bars on my Lil blue bike. Its got a cast alloy mountain bike quill stem thats top loading. The stem is meant to use a 22.2mm bars.


Dont think you can get aheadset stems for MTBs that take 22.2mm bars

Hmmm, not too many good options here . . . Thanks for pointing out that the problem is the diameter of the BMX bar -- guess I'll go with the pair of 7/8-to 1 adapter shims.

Regarding a heavier-duty stem, I'm not too concerned with weight but am concerned with strength. Any particular brand readers might suggest?

Thx.

It depends quite a lot on how you plan to use the bike. If you're going for the higher BMX bar to get a more upright "casual" riding position, then strength won't be much of an issue - any stem should do the job. It might shift out of position in an accident/incident though.
But if you know that you'll be doing more active riding, hard braking, and sort of wrestling with the bar, then there's a risk that the higher bar will rotate in the clamp. But if weight isn't an issue then just about any chunky-looking DH stem should do fine.
Strength of bike components are rarely an issue as long as the rider can accept parts that are on the heavy side.