There might be different views for looking at the differences.
However in Layman's Language S7-400 Systems are made for bigger complex and large machinery system where number of I/O's are higher and having much advanced features. Probably there will be a long list due to different parameters one will be looking at.
I can give you an anlogy which probably makes you much more clear, Like the difference in Maruti-800 and Maruti-Zen.
If you would have any question about particular S7 300 Module and 400 module then it would be easy to explain.