If you were my daughter you would not be getting a 4yr oldTB. I think young riders need older done that horses. I would be looking for a good been around the block horse. Even in the 20 age range. Somthing that has a ton of training, that you can still ahve fun on, show, even win some stuff, till you have outgrown the horses capabilities.
My gelding PJ I bought 15yrs ago My first horse. Hes now 31, we still compeat, and win. We also have several mid-late 20s horses that are great kids horses that I can grab, and go game, rope, or pen off of, and be in the top 5.
Don't over look an older horse that has been well cared for. They still have lots to offer, can teach you a thing or two, and make you a better rider without you having to worry about training them, ruining their training, or having the horse be to much for the rider.