.04 ppm is still a really low TDS. I would go ahead and identify a place to buy replacements but wait until its highher then .04 ppm. Normally you would need to change the particle filter, the carbon filter and the DI cartridge. The one thing you need to watch fo is if the carbon filter is exhausted, chlorinated water will leak through and attack the RO membrane. You may have a primary and secondary carbon filter on your unit.