Did ALL the resin change color? Because it is a gradual change. That said BRS on youtube did a nice investigation video about using your resin until the end, and as it turns out that's not such a good thing because what happens is your resin will latch onto all sorts of atoms/molecules, however each one has a different affinity to the resin (fancy speak for how much it is attracted) and it turns out those with a larger attraction can knock off those with a lesser attraction so you can flood your tank with one particular type of element while capturing others when your resin is "depleted" .
That said, if you have 4 TDS, and you're sure that number is accurate, after your RO membrane you should not be going through resin that fast at all, so double check that it all went through a color change, and if so get back on the phone to the water tech support guys.
TDS creep is a real thing yeah, but unless you turn it on and off in rapid fashion that usually isn't a problem. I've never flushed my membrane (I should though) and I make a lot more water than you before resin gets depleted, and I'm running 2 canisters of resin (soon to be 3!)
Edit: here's the video I was talking about