This is probably the hardest scenario in the original game with Mothball Mountain as a close opponent. The objective is simple and the same as Evergreen Gardens or Pacific Pyramids, so, at this point of the game, getting 1,000 guests in 4 years should not be a major problem. The main and great difficulty here is the actual building of your park, as you can't remove any trees (or major scenery objects you eventually decide to place), adjust land or water. This scenario is similar to Harmonic Hills in the Corkscrew Follies expansion, though Rainbow Valley should be way less difficult. Take advantage of rock vertical faces that there are throughout the valley extension to build underground roller coasters and tracked rides and save space. Only add scenery if you are confident that it won't disrupt the construction; once you add scenery, you can't get it off. Take advantage of the valley formation. The terrain is quite hilly and you won't find many flat areas besides the narrow river. Because of that and the main aspect of this scenario, it wouldn't be a surprise if your pathways and even your tracks get a little clumsy and inconvenient, but do your best to keep a good infrastructure.
If you set you research at maximum funding, at October, Year 4, you'll have got everything the research can offer in this park. At the beginning, you have a decent amount of good coasters, some of them being the Steel Mini and the Wooden Crazy Rodent which are excelent to start off with, as they are very cheap and compact, making your job of building around the trees and terrain much easier.
You might consider building your tracked rides mostly on the river, saving the little space available on land for your standard sized rides, but it shouldn't be a rule of thumb. Consider making roller coasters taller to avoid trees, bringing them to ground level only at the station. Another thing to note is that even if you can fit a ride somewhere, you have to make sure you can build a pathway there.
Attracting 1,000 guests over the 4 years of the scenario is not too difficult — the real challenge is constructing the park itself. Plan ride designs ahead of time and, if you need the assurance, save your game before building any large attractions. The park doesn't seem to be big but you'll find out it's pretty larger than necessary. Build the park up and attract guests slowly and methodically, and you will eventually succeed. If really needed, you can resort to marketing campaigns.