If the loud parts of the internet are to be believed, a majority of Tony Hawk game fans consider Tony Hawk's Underground (2003), aka T.H.U.G, to be the turning point for the franchise, the game where it went from strength to shambles. I differ in that I feel THPS 4 was that point; it was the title that changed the core structure upon which all else was built. T.H.U.G just took it to the next (lowest) level.
Gone are the quick, arcade style, individual runs, replaced by a more open-world, free-skate environment in which tasks or goals have to be initiated by speaking to NPCs, and to do that you first have to find the damned NPCs in the now larger environments. Making each area more expansive may sound good in theory, but in practice it means that there are less nearby objects to combo to and you have further to travel to find the next goal each time you complete one. (You can choose goals from a list in the menu once you've discovered them, but why would you want to do that?) Furthermore, having the S-K-A-T-E and C-O-M-B-O letters spread further apart makes gathering them less enjoyable.
Stat Points are now earned from completing the aforementioned tasks, not littered around the environment for you to collect. It's a process that's at odds with the illusion of freedom that the 'open-world' is supposed to give you, and to some degree it dictates the order in which you can do certain things.
The most notable consequence of the change, however, is that the huge sense of pride a player felt when achieving more than one goal in a single run is no longer possible. You have one task and once it's achieved you're returned to the free-skate-esque environment to hunt down the next one. The transition from being 'on a task' to 'off a task' is an ugly, abrupt and jarring one, making the already unfavourable process seem even more poorly considered.
I’ve saved the worst until last. Prepare for eye-twich. There are mini-games.
On a positive note, the customisation options are extensive for a game of the era, and the number of hidden, unlockable skaters is generous, so you can at least look cool in-game while you're wishing you'd picked up the THPS3 disc instead.
2½ flippity-doos out of 5