Well then life is simple - you’d only have to buy the appropriate N-pack of avast Professional Edition licenses, where N stands for the number of computers.

The pricelist is here: http://www.avast.com/i_idt_1298.html .
E.g. for 5 licenses with 1 year of updates it is $144.75 (please note that this is actually almost the same price as a 4-lic pack which is $135.80).

As about the configuration - that depends on how the computers are used (e.g. if any of them is somehow used as a file server, i.e. storage for shared files etc…).

Vlk