1. **The price of gasoline** is rising.
2. **Gasoline prices** are rising.

My understanding of the above two sentences is that they both describe a general situation about gasoline prices, not a specific situation.

My question is, if we add the word "of" or remove it, and then change the order of words in the phrase, are they still grammatically right, or can we still equally say  

3. **The gasoline price** is rising. (as compared to sentence 1)  

4. **Prices of gasoline** are rising. (as compared to sentence 2)

to describe the price of gasoline in a general sense? And more importantly, why?