Effects for only ONE section of a track

So, what if I have a track, but I only want about 4 seconds of that track to have specific effects???

split the track in that place, select the part you want fx on, and hit shift-E (it's also in the menu) or automate bypass on a track fx

Thanks 

For a fluent change, automate "wet" envelope first, than turn bypass on (it should save CPU, but not sure). Bypass curve can have any value from 0-1, but it works binary, suddenly switches in the half of the way - which is kinda logical. ;-) bypass/wet is there for every effect

if you do it the way i said, you can also crossfade the fx'ed part with the dry part, works the same way and it means your fx aren't burning cpu through the whole track.