Kamakura?
 

I have 5 whole days in Tokyo. One day is a trip to Nikko, so 4 days in Tokyo. I dont know if I should stay those 4 days in Tokyo or if I should go one day to Kamakura and spend only 3 days in Tokyo. We will go to Kyoto, Nara and Koyasan, so I dont know if it would be too many temples for one trip. Any suggestions?

So the options are 4 days in Tokyo versus 3 days in Tokyo and one day in Kamakura?

I would vote for the second option.

I really enjoyed Kamakura both times I went there. Kamakura has a completely different feel than Kyoto, etc. even though both are rich in temples. I love it's surfer/beach town atmosphere. If you are not planning to go back to Japan try to see Kamakura this time.

I liked kamakura better then nikko.