I have 4 routers on one ethernet media. Two routers works in kind of pair and should be backup for each other. Now I have dilemma. Should I use EIGRP and have 4 EIGRP neighbours or .... can I put two routers in HSRP and then use HSRP virtual address as EIGRP neighbour? What is better?
No you can not. EIGRP will form neighbor relationships on the physical (local) address and EIGRP will NOT use the HSRP virtual address.
I am not clear what your environment is and what you are trying to accomplish. But if you want 2 of the routers to talk to each other and not to the other 2 routers, there is a simple solution. Configure 2 of the routers with one AS number and configure the other 2 with a different AS number. Routers on the same segment running EIGRP with different AS numbers will not become neighbors and will not exchange routing updates.