My favourite is actually M&S they do a salted one, or some with chocolate in and their snacks are low in syns as they have a low calorie range. The salted bags are 4 syns and are a great size and the bag is full! Sainsburys also do a mulitback and they are 2.5syns a bag (i think!) and have paprika flavour too.
Found a multipack of sweet n salty popcorn in sainsburys today (own brand) - 4 syns per bag!
How big are these 4 syn bags of which you speak? Because a mate just rocked up with a stack of movies and an epic bag of them. I must have munched about 50 grams of those bad boys, they're way too moreish, I had to make her take them off me.