FIDO is 2G and 3G. There is no 4G in Canada yet.
7-11 is on the Fido network, but is restricted to 2G, afaik.
Bellus new HSPA network is 3G only, so 2G phones will not work there at all.
Also, there's more to look for than just whether it's 2G/3G. You must be sure it has the correct frequencies for the network you want. Fido/Rogers/Bell/Telus/Virgin/Solo all use 850/1900Mhz HSPA.
PS: Check out WIND Mobile if you live in Vancouver, Calgary, Edmonton, Ottawa or Toronto. You get unlimited calling for the cost of just a few minutes using pay as you go on the other companies.
(You'll need a 3G phone for WIND too, but it has to be the right frequencies for that network: 1700/2100Mhz)