Multiple Wp Install per One Db might end with bad performance?
 

do You think is it ok to install 3-4 wp per 1 db or does it lowering performance? ;)

*bumpity*

depends on several factors, like your hardware, traffic, etc. Of course, the more you pull data off the database, the worse performance. Still, if you have a decent server able to cop with the traffic, you shouldn't have noticeable issues, I'm running a 9 sites network using WP and averaging 60,000 uniques a day and sharing not 1, but 2 databases and no real problems

So meaning Your sites are using 4-5 sites per 1 db average?

No. Meaning that all 9 share the same 2 databases (it's an special setup) and then they have a DB of their own