It does seem a bit high, but if your trainer/coach/riding instructor thinks that you are ready to jump at that height then more power to ya! However, if you are doing it just to push yourself (not that I am saying you are by any means :) ) take a step back and ask yourself why you want to jump that high... If it is because you are ready, and I mean really ready, then go for it. But if it is because you just want to see if you can do it then I wouldn't. It isn't about high high you can jump but about how WELL you can jump :) just my two cents
From what I gathered from this, your trainer pointed you at a 4ft jump and said go? How much experience do you have? Was it a one time thing? Age really doesn't matter all that much, it's all about the experience..
The kind of accuracy necessary, the kind of horse necessary, and the kind of riding skill necessary for someone that regularly negotiates a 4 foot fence, to even be asking this kind of question... makes me think... hmmmm I don't think so.
I think there's a large difference between being able to jump a 4ft line or a single 4ft vertical, and it's lovely that you can do so! I can honestly say that if my coach set me a line / grid with a 4ft finish I'd be able to do it without hesitation, we've done 3'9 without a hitch so why not? That said... I couldn't navigate a 4ft course, I just don't have enough experience, and I've been riding longer than you've been alive, so I'm going to give you a word of caution- just because you've jumped a 4ft fence, please please do not try to do it again on your own. It's very different and chances are you'll get yourself hurt.