but what I don't understand is why wouldn't the other double bond undergo substitution again? They're on a carbon chain like normal so shouldn't they react with the bromine water too? Is something stopping it?
If reaction is carried out in 1:1 ratio of Br2 and 1,3 Butadiene, then the above two products will be formed. If more Br2 is taken, then surely 4 positions will be brominated.