Is #1 line returning to tank?
Is the a pump feeding #2 going through the UV? I assume so.
Any reason why you have merged 2 return pumps in to 1 line in
#3?
#4 is return line to DT, I assume.
Essentially, you have 3 return pumps.
If I were you, if your return pumps can enough flow, I would run then independently and not merge the lines in #3. If you merge them be sure that you have check valves before the merger on the lines. One return line would go direct to tank. Other return line would be plumbed through the UV to the return. The 3rd return pump in #2 wouldn't be needed.