Continue Number Series In Single Cells With Numbers Separated By Commas

I have the following syntax in B1: "1,2,3,5,6". and I need VBA to take the very last number out of that list, and increase by four in single steps. For example: before the code: "1,2,3,5,6" and then after it "1,2,3,5,6,7,8,9." The code must be flexible, though, because it will be running within another Macro, and it must work whether the last number in the list is 1 digit ("...4,5,8"), 2 digit ("...34,35,36") or 3 digit ("...111, 113, 114"). Those three examples would be changed into "...4,5,8, 9, 10, 11" "...34,35,36, 37, 38, 39" "...111, 113, 114, 115, 116, 117".