Got an offer at a tier 4 law firm (salary is just ok). I was not expecting but it happened so. Searched about them on internet but could not get much info.

Have another interview with a tier 3 law firm. Not sure what they would offer, but my gut feeling says that they might offer as I worked with them in the past. What I am afraid is would they offer something lesser than what I have received now?

I cant keep the tier 4 firm waiting as I have to decide soon but really scared to say NO to them as I am not sure of my chances at this tier 3 firm?

Please can someone give me some thoughts…