I *think* it will run at 4K res - you'd have to set the resolution as a custom one in the GEM+ Launcher that lets you run the different year/championship mods.
Best way to get GPL up and running IMO:
GPL Preservation Society installer 1.08 - adds the 1.2 patch, the Direct3D and OpenGL graphics rasterisers, the GPLEA updates for the original 1967 cars, bunch of track updates
'67 Patch 1.3 - adds support for long tracks like IoM and Targa Florio
Monaco Rocks - large graphical update to Monaco
Spa '67 - complete replacement of the Spa-Francorchamps circuit to make it more historically accurate
GEM+ installer - used as a launcher for the sim, lets you control AI strength, track lists (so you can replace Papyrus' Spa with the one above for example, and add the IoM track from the first post), which carset to run if you've installed mods, graphics settings, etc.
Now, that lot will get you up and running in the 1967 car physics that GPL shipped with. And I love those original cars. But there are alternatives...
1955 F1 -
here. Front engines, skinny tyres, enough power to get you in real trouble. Very different to the other mods. Models brake fade, which forces you to adjust your driving style to keep the brakes cool. Bloody good fun IMO somewhere like Silverstone or Spa where you can really throw them sideways without clattering into something.
1965 F1 - on the GPLPS Installer page above. 1965 cars had about half the power but ~2/3rds the grip of the '67 cars. Much easier to find the limit in.
1966 F1 - download
here, update
here. 'Fixes' some of the main issues with the original '67 physics. Cars are slightly less powerful than the originals, but still absolute beasts compared with the '65s.
1969 F1 -
here. Papyrus chose 1967 for the original GPL because it was the last year before wings started really appearing in F1. By '69 they were there in full force, so this mod has downforce. IMO, these cars really come into their own on a few circuits. Probably the fastest cars overall for GPL.
1967 F2 -
here. Great for racing, bit dull when just hotlapping by yourself. Like a '65 car but with slightly more tyre grip. Might actually be a good option for learning IoM in, I'm sticking with the '65 Ferrari because that's what I have the most seat time in.
1967 Sportscars -
here. Bigger, heavier and more powerful than the GP cars. The Ferrari P4 is possibly my favourite racing car of all time. At home on the wider, flowing tracks like Spa or Le Mans.
1966 Can-Am -
here. The Can-Am series took close to an 'anything goes' policy with the technical rulebook. The 1966 cars are quite nice to drive. But if you want insanity, then...
1971 Can-Am -
here. Hoo boy. Mere words don't do these cars justice. They have to be experienced.
Please excuse the awfulness of the recording, not only was I going from the replay mode rather than live in-sim but also I was using the Game Bar recording feature in Win10. And the Game bar recording feature is utterly bobbins.