If I attach Svellsongur to Ultrasoft poro, does it then generate 4 birds when I exhaust it?

No, attaching Svellsongur to an Ultrasoft Poro does not generate four Bird tokens when you exhaust the Poro.

According to RiftJudge FAQ #6561, Svellsongur does not double the values or output of activated abilities. Instead, it grants the unit two independent instances of the activated ability.

Here is the breakdown of why this doesn't result in four tokens:

1. Independent Abilities: When Svellsongur is attached to the Poro, the Poro gains a second copy of its text. You now have two separate activated abilities, both of which read: [T] : Play two [1] [S] Bird unit tokens with [Deflect].
2. Cost Requirement: Both instances of this ability require exhausting the Poro as a cost ([T]).
3. Exhausting: Because exhausting is a cost for each ability, and you can only exhaust the unit once to pay that cost, you can only trigger one of those two abilities at a time. Paying the cost for one does not pay the cost for the other.

To get the effect of the second ability, you would need to ready the Poro and exhaust it again, which you could do regardless of whether it had Svellsongur attached.