First of all, there's a very real phenomenon where ratings have been depressed over the last few years due to 1) sandbagging and 2) the arithmetic quirks of the USTA's algorithm, which tends to push people towards the middle of the bell curve.
That said, here's a good rule of thumb for the current state: if you go down to the courts at your busiest local public park, with the liveliest pickup scene - the best regular players there will typically be 4.0s, at best, or even 3.5 if the average level of play isn't very good.
Strong 4.0s and up will have a hard time just walking on to a decent hit. If you're not having some real trouble finding someone new to challenge you, there's no way you can say you're a 4.5. Legit 4.5s are relatively few and far between, and need to search each other out and make appointments to hit. Even strong 4.0s might luck into some decent pick-up action, but odds are they'll be at least a little bit bored.
This is not to say those are REAL 4.0s - but that's what I've found currently maps to those levels is today's USTA leagues.