Should be the exact same GTX 950M as far as I can see. I personally don't think that 4GB will be useful because the card is a entry level for gaming and you probably won't be turning up settings too often so the extra 2GB's will probably be useless. There are some games out there that require 4GB VRAM though but you can probably get by.
You can install Windows into DOS system but remember you have to pay for a windows license so add that to the cost of the laptop.
I highly suggest you get 8GBs of RAM just to future proof yourself. A lot of intensive games nowadays tend to use upwards of 2GBs and if you have 4GBs of RAM, that's already half gone.