I highly doubt 4 or 5.
I think Hewitt will be around for a while still. He will be 29 by the end of next year. He can still have a few more competitive years if he wants to keep going. Injury free, he will still be top 50.
If Guccione got it together he would be a permanent fixture in the top 100. Hopefully McNamara can improve his fitness and his groundstrokes.
Klein mighn't quite be in the top 100, but he will certainly be top 150 and knocking on the door of the top 100.
Tomic will be at least top 150 by the end of next year, too.
So yeah, perhaps a bit optimistic, but I think there is a chance we will have 4, at the least we will have 2