There is no such thing as an iPhone 4g, there are only the iPhone 4 and 4s models; the term 4g refers to a fourth generation cellular network. A 4g network, which has speeds of up to 1 gigabit per second download and 500 megabits per second upload, is faster than its previous model, the 3g, which has speeds of up to 14.4 megabits per second download and a 5.8 megabits per second upload.
There are differences that exist between the iPhone 4 and 4s models.
The rear camera on the iPhone 4s has better specifications, including the capability to shoot 8-megapixel stills and 1080p 30 FPS-videos. By comparison, the standard iPhone 4 model only has the capability to shoot 5-megapixel still photos and 720p 30 FPS-videos.
The iPhone 4s is more powerful with a dual-core Apple A5 processor, compared to the 4, which only has a single core Apple A4 processor. Both models have 512 megabytes of RAM. Capacity options on the 4s model have been increased to 16 gigabytes, 32 gigabytes and 64 gigabytes.
Both iPhone 4 and 4s models are capable of running iOS 5, iOS 6 and iOS 7, however, specific features that are supported by the models may be a bit different.