It's not bad, just starting up... mods are now undergoing development, and we should see a number of new ones start very soon. Editors for certain things are just about available, which means there should be no limits (except time!) for developing GP4.
If you mean 'will there be a GP5?'... i'm pretty sure there will be one! I'm also pretty sure that the format change won't be as significant as GP3-GP4, so editing it should begin almost right away. Hopefully Geoff and his boys can get an online license like EA has... if not, then maybe they will develop GP6 as a fantasy formula game, with online support, and with the knowledge that the GP community can easily edit it, allowing online Formula 1. You never know!