I don't think the cannon bone measurement (string test) is terribly reliable. I know in my two draft cross foals, the height it would estimate they mature to is extremely unlikely in regards to where I honestly believe they will finish at. (IE String test places Finn at maturity at 18.2 hh and Fiona at 19.1 hh. Sire of both is 18.2 and dam is 15.3hh. I honestly think they will both finish around 17-17.2 hh.) While at 4 your horse may continue to mature in height a bit, I don't think you will see too much major growth in height at this point. I think it would be safe to order your cart with a little wiggle room for some added height - I doubt he would grow enough to completely outgrow the cart from here out.