If you buy an upgrade then no, you don't get to use any more copies. If you buy LR5 outright then you can continue to use LR4 on two computers.
Seems worthwhile, especially if you wait for a sale or promotional price the new versions don't cost that much more than an upgrade. I guess if you have to have LR5 right now as soon as it releases you won't find a good price, so maybe the upgrade will be the preferred deal.
LR 4 I have seen on sale for just over $100 USD, and the upgrade cost is $79. So for about $25-30 more you could have a new copy outright, if you wait for a good sale.
Assuming LR5 pricing is similar to LR4.