As said before, but just for reinforcement, a martingale isn't going to help you.
Teach your horse to behave, don't try to force him into it with artificial training aids that won't work in this situation.
IMO, if the pony is rising 4 there is no harm in starting him over crossrails; but I wouldn't jump much higher than 18" this year. However, there's lots of useful training you can do with poles on the ground and crossrails.
I like your pony a lot, he seems like a good, useful sort with a lovely attitude.
His head carriage is quite nice, I see no reason for a martingale. The bucking may be normal green pony naughtiness, but the rearing needs to be addressed promptly. If he rears while you're getting on, he may not have sufficient balance to support your weight at a standstill OR somone might have agressively restrained him for mounting while he was being broken.
Investigate your saddle fit, and whether or not his back is sore and involve your trainer to get this figured out.
Thank you maura. Your one of the only ones who have taken this cooly.
And no, i've had him since he was about 2 and he wasn't broken, so no-one aggressively restrained him for mounting. I'll ask one of the people who are in the committee of the pony club or my horse riding coach about the rearing and how to stop it :) And I wasn't going to jump him much higher than that at all. So thank you, but may I ask what martingales are used for? Just for general knowledge.
Oh! You know what, it just might be my saddle! I was riding a quite plump little pony last year for pony club and I had to adjust the gullet plates in my saddle to fit her, maybe I might have to change them to fit Spider! That's great, thanks! I'll have to take a trip to saddle world :)