Probably more advertising than engineering, the difference is heat range.
The R means resistor to cut down on Radio Frequency Interferance (RFI) for not only radios, but other communication devices and vehicle computers, possibly even implated medical devices.
The first number in this case 4 is a thread specificaion.
The second number which is 5 for this plug is the heat range. For AC Delco lower (2) is colder than the hotter (5). The steps are about 50 degree increments and are a measure of tip temperature not engine temperature although the latter is not without influence.
The letter T means it's a tapered seat plug.
This plug is also availble with another letter S which would indicate the plug has an extended tip. These also have heat ranges similar to the non S plug. They are useful with rich mixtures, in oil burning engines, and where some performance can be picked up by pushing the electrodes closer to the bore center. They sometimes interfere with the valves when used on angle plug heads or with piston domes on high compression engines.
Other manufacturers use different systems for grading heat ranges where lower numbers may be hotter than higher, so be careful when switching brands to follow published interchanges.
Bogie
