There will be nothing wrong with using your iPhone 4 for at least the next year or two. Personally, I will be using mine until they have another great leap like I last made from the original iPhone to the iPhone 4. The only differences I can think of from the top of my head from the iPhone 4 and the iPhone 4s is an upgraded processor, 1080p HD Video recording, 8mp camera (upgraded from 5mp) and Siri (the voice recognition software seemingly only available to the 4s). If you absolutely love taking pictures and carry your own camera with you, you won't really have to worry about anything. The upgraded chip will be faster than the iPhone 4 (not sure how much faster), but I think that is only to handle the workload from Siri and the HD Video recording.
In short, don't feel bad that you just got the (previous) iPhone 4. There won't be any apps in the near future that it won't be able to run.
PS - Welcome to the forums!
- ImageX