I've realised I can't change the characters' speed now: no matter what value I write in the outfit properties tab or in the action part, the speed stays the same.
Fascinatingly, the changes do take place FOR SOME characters, but others are completely impervious to my attempts.
Another bug I've noticed is when I put an action part in the scene properties, the game executes this action part with great delay (5-10 seconds). So far this bug has only appeared in one scene, but then again, I've been on a break and haven't done proper testing since 4.0 was released, so this may only be the tip of the iceberg.
I think this has something to do with the fact that this project was created in an older version of the program (3.7). Maybe there's some data corruption on import? I'll try to create a project from a blank slate tomorrow and test that speed thing again.