Try instead, then:
=IF(COUNTIF($C$2:C2,"="&$G$2)<$G$3,IF(OR($G$4=1,MOD(ROW()-1,$G$4)=1),$G$2,""),"")
note, I also changed Row(A2) to Row()-1... same in this case...

This part:

COUNTIF($C$2:C2,"="&$G$2) counts how many times from the top of the column to the the row before the current row contain the dosage amount... this is to determine if you have hit the number of dosages allowed by the value in G3.. if the limit has been reached, then a blank is returned...

If the number of dosages counted is less than your value in G3, then the next part takes over.. OR($G$4=1,MOD(ROW()-1,$G$4)=1

the Mod() function returns the remainder when arg 1 is divided by arg 2... so the only time that is 1 is when the row() number divided by the value in G4 gives a remainder of 1. e.g. if you are in in Row 5 and your interval is set to 3, then Row()-1 gives 4 and 4 divided by 3 is 1 with remainder 1.. same if you are in Row 8, then Row()-1 is 7 and 7 divided by 3 is 2 remainder 1...and so on...