If your not going to be able to provide a bigger tank or pond for it, i probably wouldnt get one in the first place. From what ive read on this site it can be really difficult to find homes for large RTCs
rtc will grow 2ft in the first year. you will prob get a year and a half out of it. but like moo said if you cant provide for this fish i would just leave it and get somthing else
that tank will last about 6-8months if you buy him around 2". Thats also assuming you have a massive bio filtration setup to keep it healthy. These guys grow quick about 2" a month and 4x4 is a nice foot print for a baby one but as it will outgrow it quickly U should be setting up at least a 500gal or a indoor pond when you get him. Generally by the time u have the new tank set up and cycled hell be ready to move. And dont buy one unless your planning on housing him for life.