This is a strange one but it's something I've encountered before. When ever I go past a certain year, either it being year 3 or 4, the game will either lock up or completely crash. On my disk, it will crash on me, but with the installer files from GOG, it locks up once I enter year 5.
any ideas around this problem?

I figured it out, you're going to love this.
While playing, I was having consistent good luck with scenarios not crashing the game when I completed my objectives, all while having an insane amount of guests in the park, but, I came across this issue in a scenario I have never had it happen in before.

Speaking hypothetically, what if you had a park that has approximately 3500 guests in it, and seeing that when you complete a scenarios objectives, they all clap their hands.
Now, say that 80% of that 3500 are all holding a balloon of some colour or another., you now have 2800 balloons being released into the game world at the same time by several different entities, this is actually too much for the game to process, causing it to crash.

So, I figured it out!

An update, I replayed through the same scenario that crashed on me (Leafy Lake) I did about the same thing, with some differences of course, resulting in me finishing the scenario and it's objectives, the game did not crash, which concludes it was in fact the balloons causing my crashing issue.

The reason for not testing this sooner was the fact that I used to throw balloons at every park in mass because I liked hearing them at the end of a scenario when I won, this however became my downfall the better I got, which I was too good for the game, or at least, I was too good at distributing supplies from shops to my guests.

So, yeah, problem officially solved! To anyone having this same problem, just don't use balloon stalls at all in your parks, or at least limit it some, you should be alright.

I did know that the game slowed down at about 3000 or so guests, it seems the engine is even more limited than it's commonly pointed out. Thanks for sharing your experience :)

Well, it's not so much that the game is slowing down, it's running just fine for me with that many guests, though results may vary on what kind of hardware you've got going for you, I just never thought about the balloons having an issue until recently when I had a play through of Evergreen Gardens where the balloon sales were garbage, compared to a Leafy Lake play through where the sales were outstanding.
So, it's not the guests, it's the balloons. The game can't handle too many released at one given time.

It might be the number of sprites (i.e. the number of peeps, baloons, cars, birds and everything else) that causes the problem.

Two thumbs up for the great problem analysis. :)

That's very possible it is a sprite overload, guess I have to sit down and attempt to range in 6k guests into one park to see if that's really the case, I know just the park too!
I could've honestly figured this out faster, but I was convinced at the time it had something to do with year 4 into 5.