Question Help! How to auto-timestamp multiple fields (Is not null)

[Ignoth]

I am trying to create a tracking system, and need to set 4 different fields to be auto timestamped "Now() " after those particular fields are filled/modified within each record without affecting the previous timestamps. Also would like to run aging reports based on those records. I just have 1 master table, I'm not sure exactly how to go about doing this.

(i don't know VBA either)