I don't know, most of the 4.5s I've come across are guys in their 30s who have been playing since high school. Many are former college players. It's a pretty high level.
Former college players doesn't mean much to me. There is a WIDE range of skill in "college players". Even more so if you include bad ones not playing for a long time.
If we're talking D1, they have to be like in their 40s and have not played in quite a while to drop to 4.5.
I played at roughly a 4.5 level and started HS Sophmore. This is with 0 private coaching.
And you can be a 4.5 player who looks like he hits like ****. Most people will never say "he looks like he's a high level player", proceed to get double bageled, and continue saying it and following up with "he got lucky when he beat me, I made a bunch of stupid mistakes". Fact is, you just need to be intelligent to win. That's all. It's competition, it's not about strokes, it's about the mind, the desire, and the determination to punch a hole through any wall that gets in your way.
I'd guess a fairly fit 30 would be the cutting point. Think of it this way, once you hit 40, you should give up on improving your NTRP. Once you hit 30, this are mostly going downhill from there, but if you maintain your fitness and improve the way you look at matches and how you execute, you can still get better. But by 40, I don't see any way you can get any better.
I try not to underestimate the abilities of people. The majority may not be capable of much, but some stubborn and/or talented people have a way of just breaking the rules and surprising you. There's a guy in his 90s with a winrate in the 90s because he moves better than his opponents. Crazy **** happens. If you aren't talented, work harder than those who are talented. If you aren't determined to go through hell and back to reach your goal, you probably won't reach it if it is a difficult one. I'm not a very determined person, so I don't get too far into anything I do.
I'm going to guess you're in your mid 20s. I'm telling you now, if you REALLY want to be a 4.5, you can do it, but you'll have to really invest into it. In the end, you may end up hitting 5.0 before you realize it. 5.0s are good, but they're not particularly amazing. I mean, I'll get thrashed in every match I play against one, but it'll never be, "no matter what I do, I'll never be at their level." I can get there, I just don't care to invest the time, effort, and resources into it.
If you want to guarantee improvement when you feel like you can no longer improve, you need to 1) get a trainer and improve your body with tennis as the focus (at the very least, you need to hit the gym and go run some miles/kilometers outside), 2) get a good coach to constantly keep you on the right track to improving properly, 3) get a good hitting partner to keep yourself motivated and keep you used to dealing with quality shots, and 4) constantly practice and stay motivated. I was not willing to do 1 and 2. And I got more fun out of hitting the ball than spending the time to meticulously improve my shots. I guarantee, you have not done all 4 of these things. I would say #1 is has the most possibilities for improvement out of the 4 for most people. If you believe you have more stamina than your opponent, it's a mental boost. If you feel like you've worked harder than them, it's a mental boost.
tldr; you can do it if you believe and work hard for it;
1) go to gym, talk to trainer
2) get good coach
3) get good hitting partner
4) keep practicing
5) f*ck *****es, get money! Errr, I mean believe in yourself, stay determined, and keep enjoying tennis. The point where you stop enjoying tennis is when this all becomes meaningless.
