We are planning to change to a new provider that will be cheaper, and we were wondering if a 5 Mbps connection will suffice 4 users at once? Also... 3 of us use it for gaming so we need to know if it will be fine.

I work for a company that support bank networks. A couple of our clients only have a 5Mbps connection for about 15-20 users, and it seems to work pretty well for them. So I'd say you should be fine.