newborn baby

so since I've left the hospital my baby has been on similac pro sensitive. he is 4 days old and is drinking from 1-2 oz every 2-3 hours. he hasn't pooped since Friday and I see doctor tomorrow..(called doctor this morning and increased the hours in between feeding since hospital said 3-4 hours) do anyone have this happen with their newborn does this mean it's his formula? I bought a huge can of powder since he is about to be out of ready made. I dont want to open it if it might be formula??

also is there a difference in powder to ready made?