I'm assuming you are using particles stuck to the object they are emitted from, is that correct? If so, the fact that it worked at all in XP3.5 was the result of an error in the emitter. Once that was fixed, this method no longer worked. Instead, we have the 'world speed' in XP4, which is the speed particles have when stuck to an object. You can test for this in the question object, for example.
I don't know what you wanted to do with the calculated speed, but hopefully the world speed will do it for you. If not - if something still does not work - report back here and we'll take a look at at it. You can find the world speed documented in the manual in the emitter extended data tab page.