I hate to say this... but there is NO way to keep a 4 year old safe on an open tractor.
A previous minister at our church lost his little brother when he fell off the open tractor and was run over by the disk. It happens. And will again.
I have 3 boys of my own... and its hard. I do take them on the sound gard cab tractors, and probably shouldn't do that.
Don't mean to pop the bubble... but you could probably buy a new tractor with a buddy seat for less than the accident cost.
Please be careful.