I was also having this issue after updating to 4.3. I posted here and got no responses:
I ended up uninstalling 4.3 (deleting from /Applications) and installing from the 4.2 installer to 'downgrade'.
I've experienced exactly the same problem.
Constant crashes when using the adjustment brush in Lightroom 4.3 on a Mac Pro running OS X 10.7.3 Lion.
The application just quits, no error messages.
No problem at all using it on a Macbook Pro Retina running Mountain Lion.
Have deleted 4.3 and reinstalled 4.2 on the Mac Pro, which is now working fine.
This has been happening to me since updating to 4.2 and then 4.3........ 4.1 had no problems at all and ran well on vista 32 bit.
I even tried using less strokes with the brush to see if that would make a difference, and I thought it was at first, but it crashed again.
The only error message I get is "Adobe Lightroom has stopped working" etc. Which wasn't very helpful.
Is there a way I can downgrade to 4.1 (the default version that came with the cd) without losing my databases?
My OS is 10.7.5 and I have the same problem. But only when using brush tool after applying a redeye fix. it crashes immediately every time you even atmept to do it again.
I also want to mention that 4.3 is significantly slower than 4.1 that was shipped with my DVD disk
I've downloaded the 4.4. It's snappier. It doesn't crash as instantly... but it crashes eventually
I have no problems with the brushes. Mine crashes with any photo that I apply redeye tool. And if I manage to apply it before it crashes.. this file becomes unusable; meaning it cause Lightroom to crash.
This version allows me to more time before it crashes.
I did tried with a new catalog.. no use.
4.2 doesn't have any of these issues even though it's the slowest of them all
PS: Running OS 10.7.5