Well hand held digital scanners are wide band (VHF, UHF, 700/800mhz) they can decode different protocols, Analog, digital conventional, digital trunking, some with upgrades can do DMR and Provoice.
The G4 is a 700/800mhz receiver only. It will do analog conventional, digital conventional, Phase 1 trunking only. No DMR, No analog trunking. No phase 2, yet.
G5 is same as G4 but it has a second band. Can be a VHF / 700/800 OR several different splits of UHF/700/800.
G4/G5 handle trunking reception much better. But lack field programming capabilities, lack hold and lock out features, were designed for first responders and picked up by scanner people due to the good RX capabilities.
Each device has its own pros and cons. Learn those and see what fits your particular needs.