The cable modem of my ISP is restricted to four devices. Is it possible to avoid this restriction if i connect a WIFI Router with own DHCP server, static IPs etc. to the cable modem so that I can connect multiple devices to my WIFI Router which uses only one position of my cable modem?
Yes, if you have a wifi router plugged into the cable modem, the cable modem thinks only one device is connected. Additionally, all wi-fi routers (that I know of) come with a feature which mirrors the MAC address of your current computer so that the cable modem thinks it is just your same old computer still connected, not a new machine of any kind.