Four months? Maybe time to change pre-filter(s). Although they usually last longer, the higher TDS MAY be a sign that the pre-filters are starting to clog slightly and reduce pressure to the membranes. Better sooner than later as eventually, they may let enough through to harm the membranes irreversibly. If that happens, you will notice a much higher TDS along with a big drop in production. I go 4 - 5 months between pre-filter changes. Mine are 20". If yours are smaller, they ought to be changed immediately.