I doubt you'll miss many places on the waiting list if you wait to buy a membership for next season once the memberships 20/21 are available. Buy one now and you'll be renewing again in the summer. From my own experience of my daughter being on the waiting list (but we didn't take up the ticket...
4 is still very young to be going to every game. Probably far better off getting a ticket for the odd game, then seeing how he likes it as he gets older. You can always buy a pair of seats in the Family Stand for games and transfer your ST seat across.