The real answer is will you be allowed to demo / see it before you buy?
If yes, then it might be worth it, check the lens carefully and look for bright corners. This stuff is still somewhat of a gamble with how good of a sample you get. If it's a bad sample, then $4800 is a total rip-off. However, even if it is a good sample, I probably just get the newer JVC rather than risk it, but depends.
$4,800 is a bit high for a used 990.
Personally, I'd offer no more than $4000 for the used 990, and that is ONLY if I can see it, otherwise the 790 is basically the same projector and you have better odds of getting a good sample with a new JVC rather than used. Used projectors (including 'supposed' floor models) are often ones that were returned or ones that had an issue that the dealer decided to keep from a return to use as a floor model. I know this because I have seen several b-stock and returned JVC's, and many many of them had lens issues. So be very careful buying used JVC's, as people love to flood the used market with bad samples. It's almost more dangerous to buy a used JVC from a dealer than an individual believe it or not.
700 hours is nothing for a JVC, they generally last 15k hours or more. I know several people that put 10k to 15k hours on it, I am approaching 10k hours on an older one myself.
If it were an LCD, I would say 50 hours is too much