You don't need to burn a very big forest, but a forest with loooong frontiers to prairies. So try to organise the that on stage 1.
Also I try tp keep most of the initial trees out of the forest, so I can grow many flowers between them
Good advice.
You can also burn a huge forest and rebuild some of the toxin scrubbers. Then you can plant some chunks of forest and put lots of greenery in between.
I found area III harder than area IV. After I got past the flowers, I failed the stage twice because I could not get the weather going. The steam generators need a 2x2 space of water with power and most of the good spots need some channels for the water and then you need to build power on top of that. This is really expensive on top of the already expensive steam engines.