I think as a parent myself, if my child came to me with a well laid out plan of how he/she wants to help make money to get an upgrade for his/her pet. You would also need specifics on how your turtle needs more room to be healthy.. Make it like a project/proposal. Look up info on your turtle how it needs ten gallons per inch.. Look up how big he/she is presumed to get, and how it would be cost effective to just get the tank that will be big enough for it to live the rest of its life out.. Also put in your project how old your turtle can get. You can also, pull on heart strings about how you are just trying to be a responsible pet owner and give your pet what it needs.. You parents may have no idea how big or long your turtle is gonna live in good conditions. I had no idea about turtles till 4 yrs ago and I am 44. Also, as Steve said come up with a plan to help pay for the tank,, at least half of it anyway. YOu could research what a new one costs, but also look on Craigslist, there are many good deals on there for a tank of that size with a stand.. You are still looking at probably 200-300 dollars anyway. I would also mention in your proposal/Project that for four years you have been doing a pretty good job looking after this pet, and that its very important to you to give it a good quality life.. Stuff like that,, I don't know how any parent would not be impressed with you putting so much effort and thought into it and be impressed with your maturity.. But most impressive to them I would say is that you have a plan to help with the money. As Steve said, anticipate gifts and possible extra jobs you can do to make a little money. I used to cut grass for money and shovel peoples driveways in the winter. If you asked your neighbours if they had any yard work needing done cause you are trying to save for your pet turtle to get it a bigger habitat bet most people would find something for you to do..lol Not many kids are willing to work at all anymore and to have one who wants to do it for his pet. Yeah I would probably give you the money without finding a chore for you.. lol
My daughter got a ferret I didn't want because she put a lot of effort into planning for it. I bet it might work for you and your turtle..
Goodluck
Midland Painted--Philbert--SIP
1-2 1/4" Mississippi Map Turtle(Male)--Atlas
1-2 1/2" Western Painted Turtle (male)--Titan
40 gallon tank