Well that's were am stuck a bit, not sure where or wen I fill ever up it's different all the time, I do a tap tds test an one after I have ran the RO for a few mins and as long as it's in the teens al fill up.
So am not sure but is it hf4 for low pressure and hf5 for high ....... But I won't be using a pump
I do like the 4021 but the 4040 will fit along the front of me tank nice