For starters, the 3GS came out exactly one year after the 3G, not a year and a half.
As for a 4G iPhone, don't even think about it for several years. The 4G technology being deployed by Verizon and AT&T is called LTE. It's still on the drawing board at AT&T and is currently in testing by Verizon. We might see a 4G Verizon network late next year, but only in major metropolitan areas. AT&T is probably going to start testing late next year or more likely in 2011 with deployment coming late 2011 or 2012.
If Apple has a five year exclusivity plan with AT&T as it was initially rumored, then no 4G hardware until 2012 (first iPhone came out in 2007), which would be in line with AT&T's deployment schedule. Now if it's only three years, as some would speculate, that would put the end of the contract at next summer. However, that's still too early for even Verizon's LTE network. As such, the next generation iPhone, if it's released next summer, will NOT be 4G enabled as the network simply won't be there. I suspect we'll be waiting for the fifth (2011) or sixth generation (2012) until 4G hardware is built in.
If you hear people saying that the 4G iPhone is coming out next summer, they're referring more to the fact that it's the fourth generation iPhone (3G was second generation, not third generation as the title might imply; the 3GS was third generation), not that it's going to be using the 4G network.