I have to give a hard disagree with that. GT1 and GT3 make a lot of use of their content (in this case, cars and tracks) because there's a lot less of it.
This however doesn't mean GT4 doesn't do the same. If anything, it just gives you so much choices that you end up never using much of the content due to how much volume of it there is, that however can never be viewed as a bad thing.
Yes, less content makes it easier to use more of it more fully, but that doesn't disprove that GT3 uses it's content far better than GT4. More content doesn't = a better thing by default, too much content with little to no direction can mean a game comes across as a convoluted and confusing mess. That's not to say GT4 is a convoluted and confusing mess, it isn't.
It's very tight between GT3 and GT4 for me, but I 100% enjoyed the structure of GT3 more than that of GT4. My point isn't how much content you use, it's how much content the game makes good use of. There are plenty of cars in GT4 that have no race they are competetive in, there are plenty of cars that are mere duplicates of one another, there are plenty of cars that are only competetive in a single race. It has a lot of content, but it doesn't make great use of a large chunk of it regardless of how much there is in the game it does make use of. GT3 is far better structured in that regard.
GT4 is regarded by most people as the best, I'd probably edge it over GT3 myself as the better game overall, but there are plenty of people who prefer GT3.
GT4 basically has all of the content GT3 has but... more... It really can't be considered worse.
It can, content alone does not a good gameplay experience make. GT7 has plenty of content, but it's absolutely a worse game than GT3 IMO, it has a lot more content though.
Aside from the odd car here and there like the Jay Leno Tank Car and the 1hp 1800s Mercs, what other content in GT4 is not propperly used? Basically everything else is, but, like I said, with a ton more choice.
There are a lot of cars that natively don't perform well in any given race series, IIRC none of the GT300 cars have much use, the touring car spec race cars don't have a use, cars like the Lupo GTi Race Car and Beetle Race Car have no use. There are a lot of cars that are in the game because they could be and not much more.
A lot of the race series have a limited number of halo cars that can enter and are by and large needed to win, else you have to tune cars. Now that's fine, tuning cars to perform better is a key part of Gran Turismo, but GT3 gave most cars a series they were competetive in stock as well as tuned.
A big reason for that is the fact many of the series repeat across the Beginner, Amateur and Profesional levels for example, the Beginner MR series features low powered MR cars, by the time you get to the Professional MR series you're racing high powered performance MR cars. I don't doubt from your posts you prefer GT4, but there are plenty of positives for GT3's career structure over GT4's. I categorically prefer the structure of GT3 to GT4 and I'm not alone, but overall as a game I tend to pickup and replay GT4 more often.
The use of the tracks in the game is almost perfect, and now instead of just having the 787B as the sole Group C car in the game, you have like 5 or 6 others to choose from, among other car categories where GT3 simply fell short by comparison.
EDIT: Other than the 100+ duplicates of course... But even with those, there's still 500-600 unique cars of choice and over 95% of them are usable.
GT4 nails the track list, it's arguably the best in the series. I'm not knocking GT4 by the way, it's arguably my favourite Gran Turismo, I'm just knocking the notion that GT4 is better than GT3 in every way and therefore the suggestion that no one should prefer GT3 by extention.
