I have 4 computers (in a radius of 50 meters) connected via switch. all have them use internet (modem>switch)Now i want to know wether i can use a composite wifi (wifi with 4 lan port) with same speed of internet or not. i will remove old switch.
Not sure how you made it work with just a modem and a switch. This would mean each of your PC had a IP address from the ISP. To work otherwise your modem is really a router.
If you were to cable you machines to the router like you did the switch you should see similar performance. The lan ports on "most" routers are actually a small switch and run at full wire speed. Traffic from the lan ports to the wan port uses the main processor and could slow you down...maybe. It is more likely that the router is many times faster than your internet connection so that will be your limitation.
Now if you are asking if you use wireless instead of wired. That you will likely see it be somewhat slower. How much is going to depend on the type of traffic and where the traffic is going. It is very easy to completely saturate a wireless network coping files between 2 machines in the house where a wired connection would have no issues.