I'm not sure what you're asking. Surely all versions of the S5 are 4G-ready? Perhaps you mean the setting, in Settings, More, Mobile Networks, Mobile Network Type.
The network you're using (the carrier) also has to be 4G-capable. Some of the smaller ones may not be, so even if the phone is, you won't get 4G. (It could also be a case of a phone made for a different carrier, so you don't have a radio on the frequency that your carrier uses for 4G. We don't have enough information from you to narrow in any further - like the make and model number [not S5 - G900<something>] of the phone and the carrier.)