I am just starting out like you, but here is my take…
I have put about 6 hours in on my 2-line 2m trainer, and I feel like there’s not much more to learn. Yesterday, I was out, and I was able to carry on a conversation with one guy while not looking at the kite, and while figure-8ing the kite on one side of the wind window. I then was able to do the same on the other side, while not even really thinking about the kite and while watching the one kiter on the water throw down his tricks. This was in gusty 12-24 conditions. I’ve also simulated water starting and falling down and maintaining control of the kite, while not looking at it. I can also loop the kite, and dive it to near the ground and bring it back up without crashing it. I can fly it with one hand in lighter winds, though this is difficult without a chicken loop in higher winds. I’m not sure what hours 7-20 are supposed to bring.
Despite all of this, I have no idea how depowering a 4-line kite changes things like where the kite flies in the wind window. I am not able to practice or simulate any sort of self-launch or water relaunch. In addition, there is no chicken loop on my bar, so I can’t practice hooking in and out and feel that the water starting practice is lacking something. If you have the money, I would say that perhaps the Ocean Rodeo Rise SLE 2m trainer is the way to go, though. However, at ebay, the price is $330, and that’s an awful lot for a kite you’ll probably use for 10 hours. I haven’t seen the bar.
Another option would be a use real 3m or 4m kite (perhaps you can get one used), though at your weight, you would only want to take it out in super light winds. It could perhaps double as a high wind kite once you are on the water, and you could probably use it on snow, though I don’t really know anything about snow kiting and the size requirements. If you just want to get dragged across the snow, you may not need a very large kite. I only weigh 125lbs, and my 2m is able to give me a decent yank when I power it through the wind window in 20+ winds. You could practice during the winter, but I don’t know what you are intending to do with it on snow. HQ sells ram air kites that perhaps could be used for this purpose and that go from 3-5m. You can even get one with 4 lines. http://kitemare.com/trainer%20kites.htm#HQ_Scout_3m
I’d say using a trainer kite has been useful (and it is fun), though you will reach the limits of its usefulness pretty quickly. Also, I’ve learned a lot from watching the Progression DVDs.