I don't use APPS2SD at all as I don't use that many apps. However, I think it is important that you understand more about your last question rather than just receiving a "Yes" or "No" answer.
To answer your last question though, Class 4 merely refers to a very specific type of speed rating. Assuming a fully fragmented disk (Fragmentation should have 0 effect on solid state media), the class refers to the maximum sustained write speed of the disk. Thus, your Class 4 card can write data at a rate of 4 MB/s.
Apart from the fact that apps are generally small to begin with (Thus you are unlikely to notice any I/O occurring outside of loading larger files like pictures), the 'class' has no bearing on read speed. With this in mind, running apps from the SD card deals with read performance - something that is unaffected by the class rating.
All of this said, the class rating is a speed rating and has no bearing on whether you can use A2SD or not