4BLD and 5BLD are challenging. If you enjoy blindfolded solving and have a good handle on 3BLD go for it, otherwise I suggest learning to speedsolve first. The standard order is to start with 4x4 or 5x5 first, and then move on to 6x6 and 7x7. I learned 5x5 then 4x4, because of parity, but in hindsight the 4x4 parity alg that is different from 5x5 parity is very short and simple. My favorite is 6x6, and if that's the way you decide to go the MGC is miles ahead of any other 6x6 on the market. I would recommend starting with 4x4 or 5x5, doesn't really matter which. The ones I use are discontinued and heavily modified, I can't say which would be best to get.