There is a feature on IPB 4.0 that is bugging me and my users. When a user posts two comments in a row in the same thread within a certain time frame, it merge's the two posts.

So can it be disabled, I've searched through Admin CP and these forums and cannot find a way to disable it.

If you type merge in the search box in the upper right in the AdminCP it will pull up the setting.

It certainly can - in your AdminCP, look under the System > Posting section for the 'Merge Concurrent Posts' setting, and disable it there.

We've changed this setting to be off by default in our next release, since we've noticed it causes some confusion

I scrolled past that setting around 10 times.