I had a tooth extraction 4 days ago, i'm still in a bit of pain and there's now some yellowish tissue in the socket. should i be worried?

You will live: totally normal and you will be just fine. the mouth is a moist environment and the site/clotted area will take on many colors as it is healing (yellowish being one of those colors). Just keep it clean;)

It is Normal: Having mild pain and yellowish tissue in the socket are normal signs for recovery. Granulation yellowish tissue is good sign, it means the extraction site is healing properly.

Post extraction: What you describe sounds normal. However, the dentist who extracted your tooth should follow up his/her care of you with a post-op visit to confirm this. Call and make an appointment if you don't already have one.