You're coming to Umeå? That is so cool! Welcome
Ponies are a bit scarce. If you mainly want G4s there is a toyshop down in the city centre, as well as two supermarkets; Coop Forum on Ersboda and Ica Maxi on Strömpilen that has some ponies. I'm not into G4 but from what I've seen it's not a lot to cheer for. The Red Cross (Röda korset) and the salvation army has second hand stores where you might have luck to find some as well. The Red Cross has a store in the city centre but it's mainly cloths, for other things you have to go to their larger store in Mariedal. Salvation Armys store is called Myrorna and is located on Ersboda not far from Coop and it is a walking distance to Mariedal.
For transportation there are buses that goes pretty much everywhere. Number two goes directly to Coop, number five to Strömpilen and 8 to Ersboda. Tickets are priced ok and you can pay with credit card direct on the bus, they don't take cash.
From what I know there is not a Disney store in Stockholm, but in Old Town there is a shop that sell pretty awesome christmas ornaments and they have extremly nice (and too expensive for me) Disney themed ones.
If you want to know anything else about Umeå feel free to PM me.