I updated LRT4 today, and the first time I tried to export a sequence nothing happened. I solved the problem by browsing for the LRT4 executable again in the export dialogue, so that seems to be the workaround.

I thought you should know!

Maybe it was still set to the LRT3 executable.

Not in this case. I had that issue when I first upgraded from LRT3 to LRT4 and I visited the forum then and used the suggested solution to fix the problem.
The issue I'm reporting here happened without LRT3 on my machine, after I updated LRT4 to today's 4.0.2 update.

I closed LRT4 while I installed the update, but I'm not 100% sure I closed Lightroom. Could that have been the problem? I have Lightroom CC 2015 on trial licence, by the way.

I don't know, but at least it works now :-)