In short, there will be no difference between a device labelled SN74xxG4 and SN74xx.
Historically, G4 and E4 suffixes meant that the devices were rated to be "Green" or "Lead Free." However, as time has passed, all devices have been pushed to these new "Green" or "Lead Free" standards, but the part numbers have remained available so that customers have had a consistent orderable part number.
To check on the "Green" status of a part, you can put your part number into the search at the following link:
For definitions of E4, G4 suffixes, please refer to the following:
You can also refer to the end of the datasheet in the Part Orderable Addendum to see the "Eco Plan" of a specific part orderable.
The electrical specifications for every device listed in the orderable addendum is that listed in the datasheet.