BeerBorg

If my plastic beer bottles get really hard after 3 or 4 days should I burp them and leave them out for conditioning or should I move them to refrigerator for conditioning

more than likely ... if the PETs are rock hard then they're carbed and ready for the fridge.

i might be a bit concerned about them being rock hard so soon.
could mean they were placed in a warm location and finished carbing in half the usual time.
could mean they were bottled too soon or too much bottling sugar was used.

PETs are rated for pressures higher than most beer bottles. 10 vols CO2 IIRC
keep an eye on the caps and the bottoms.
if they start to bulge you may need put them someplace easy to clean up ...

The only reason it would be "bad" to fridge them is if there is noticeable acetaldehyde (green apple) lingering in the beer since it hasn't had much time sitting on the yeast. That will *never* clean up (go away) sitting in the fridge, it has to be warm with the yeast still working for that to happen. If it has that, and your pretty sure it's going to over carb in the bottles, then burping would be better then fridging right away.

You might want to put ONE in the fridge, and then try it and see if it tastes good to you. If so then fridge them all. If not... maybe burp.