My one month old son goes at least 4 days without pooping, is very gassy and fussy and when he does its pasty and dark green. normal? (formula fed)

Yes, normal: The stool you describe is normal formula fed baby poop. Going 4 days in between is a bit long, but not unusual. More concerning is his "gassy and fussy". Make sure he is not feeding too fast and thereby gulping air. Make sure you burp him after every ounce of feeding. You may also want to consider one of the many formulas that promote themselves for "gassy" babies. Talk to pediatrician for options.