Define premium. Because the game doesn't.
A car modelled specifically for PS3 (and future) hardware from the ground up, made for GT5P, GT5 or GT6 in full HD graphics. Having a fully modelled interior is not what makes a car premium but rather being a premium is what gives a car a fully modelled interior. Of course there are exceptions to this as mentioned below.
What defines a standard is a PS2/PSP model carryover that was not originally created for GT5P/GT5/GT6 and was not created from the ground up with full HD graphics (in fact, most are straight from PS2, untouched). Many premiums have been upgraded, with some parts of certain models looking very close to premium quality -- these have been labelled "semi-premium" around here, although at the end of the day they are still standard models (PS2 models with facelifts).
So the M4 is premium? No interior, can't be painted, can't be modified... seems standard to me.
The M4 is a premium as it was modelled for GT6 in full HD quality. It's not a PS2 quality car carried over and is therefore not a standard. It has no interior but will in an update, but regardless that doesn't make it premium or standard. It has full body panel separation, fully modelled headlamps, and was clearly created from the ground up in HD graphics.
The GT-R Proto from GT5P is another example of a premium without an interior. Its exterior is clearly the same quality as the normal GT-R and it only lacks an interior because the car was a prototype and had no designed interior to model. It was also definitively listed as a premium in GT5.
Also, lacking customization options does not define whether a car is premium or standard either. Premium race cars can't be painted or modified and they're certainly not standards.
Still isn't premium. That's like saying the Countach was a premium in GT5, because it would later on become a premium.
Nope. The Countach in GT5 was a carryover from GT PSP, which effectively made it a PS2 standard. It was actually a bit better than PS2 graphics (having been modelled well after GT4) and thus it was one of the best standards in GT5, but the Countach was completely re-modelled for GT6 and is a fully premium car now. Its exterior is clearly superior to that of GT5's standard Countach.