The filtering system GXWHO4F came with no directions.

http://waterheatertimer.org/pdf/GE-smartwater-GXWH.pdf
Optional accessories are available (Visa,
MasterCard or Discover cards accepted) by
visiting our Website at ge.com or from Parts and
Accessories 800.626.2002 (U.S.) or 800.663.6060
(Canada).
Maybe they got them, maybe they don't.

Absolutely feel obligated to upvote the free help.
Or take advantage of fixya paid help.
For a price, expert works with you while you work on filter or any do-it-yourself project.
Fixya is always less expensive than a service call.

Filters always reduce pressure.
You can add a pressure tank between filter and tankless.
Tankless are sometimes like bowling on a ship.

Add a comment saying how you fixed problem.

Fix most tankless problems yourself, same day, all brands for $330.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Absolutely feel obligated to upvote the free help.
Or take advantage of fixya paid help.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

This is proabably not the case but I have seen it happen, Have you checked the outlet side of the filter to make sure it was drilled when it was made. Thank you

Pull all the filters out and re-install them. Let the faucet run for a while to purge any air from system.
RJ

It sounds like there is quite a bit of sediment in the water. The filter you are using may be a very fine micron 5-10. Try using a 30-50 micron filter.
RJ

To replace the filter cartridge. Do the following;
1. Assuming that you have a pump in your water filter
system, shut it off.
2 Close the intake valve before your filter system.
3.To relieve the pressure, open the faucet after your
filter system. Any of your faucet for that matter.
4.Once the pressure is relieved, you can of course
replace the the filter cartridge.
the filter cartridge

I don't believe that any filtration system is capable of removing dissolved calcium from water and that is what causes the white deposits. Water softeners will, but I don't like the idea of dissolved salts in our tap water.

it's just a cup screen,, when you take off the cap screw you pull it out claen i and put it back,, it is on the cold side only right before the unit connection

The red button is a pressure relieve valve. If the system is leaking from the valve, try pushing the valve up from the underside of the cap where the valve is located. If this does not work the housing needs to be replaced. You do not have to replace this with a name brand like GE. All you need to do is make sure the housing that you are purchasing is the same size.

Example of Replacement Filter Housing: http://www.freshwatersystems.com/c-107-filter-housings.aspx

FreshWaterSystems.com: The Water Super Store: 1-877-335-3339