Recently bought a 4pls and r204gf-e recievers and was told the recievers wont work in 1/8 buggy/truggy or 4ws sct, is this correct?

The internal antenna can give a shorter range compared with the wire antenna.
The real advise is not to use it on large tracks although good range experiences are reported.

I used those receivers outdoors; never ran out of range or had any other issues, YMMV.

But can i use them on 1/8 vehicles ? I was told no

The only reason not to run that rx in any car would be range related. It doesn't know what scale car it's in, in fact, those were initially marketed to small scale flyers and car people started buying them in droves.

You already bought it right? Do a slow range test and if your track isn't too big, you'll be good to go. If it failsafes on a signal loss, they're easy enough to sell/trade for a different rx.

They say that because 1/8 is mostly driven on larger tracks and electric 1/10 on smaller tracks. As mentioned before the actual warning is just a range issue that can (!!) happen but there are many stories that there is no issue.

FWIW, the published range on that rx is 230ft. Given how conservative Futaba is known to be with those numbers, there aren't many tracks it wouldn't work on.