This is just me, and my limited experience with them. I think you will find the newer one, 4018, to be a strato type saw, with the secondary air inlet on the manifold. The older 2375 is probably the traditional setup. And, the one I much prefer. But probably won't notice much difference.
I don't know the reasoning for the different model numbers, I think they just changed the model numbers after a few years, like on other saws they made over the years, and after minor changes, like the 5200, 5400, 8500 for example. They are all 85cc saws. The 2375 & 4018 are both 42cc saws.
The older ones, some will have a chain brake, but some not. I prefer them without myself. I think all the newer ones have one. My sisters boyfriend has Wild Things, thats all he uses.
Buys them cheap and runs them till they are plumb wore out. He has cut tons of wood with them over the years.
Thats how I got the 2375 I have. He got several of them for nothing practicaly, both had bad P/C's. I put a new P/C in the one he wanted, and I kept the older 2375, bad P/C and all..LOL Thats the one I use for the Wild Thing races..
Makes me wonder how it would run with a New P/C.
Gregg,