There are pros and cons to both projectors. I have seen the big brother to the HC4900 (the HC5000) and was not impressed compared to the PQ of the 7210. There are many reasons why and they have been discussed before. Apart from the 1080p resolution (that is if you will notice it), the HC4900 really cannot compare to the 7210 in PQ.
Other items apart from PQ to consider are dust blobs, placement flexibility, noise, lens quality, brightness, processor, scaler, iris etc. They vary between these two units.
The way I and some others have looked at it, the few pros and the negatives the HC4900 gives you is not worth the $500 difference by any means.
Do some searching around here, read the reviews of these two PJs and come to your own conclusion would be my advice.
Good luck.