When DD2 turned 8 last July, we bought her a selection box of sweets for her birthday. She was allowed to keep them in her room and eat or share them as she saw fit. The rules were no bribing her siblings with them and no gloating, or they would be put on top of the fridge and be deemed communal.
It worked well, she shared some and kept some and it was a couple of months before she'd finished the lot.
DD1 turned 10 the week before Xmas, and we did the same thing. She still has 2/3rds of them left as she's a lot less likely to share them than DD2.
This week DS turns 4 and I think he's a bit too young for a similar amount of sweets. Would you treat him the same as his sisters?
DD3 is 6 in about a month and I was planing to get her the same box as her sisters. She wants them and is looking forward to them, but I can't decide what to do about DS.