I had an aftermarket remote start unit installed a couple of years ago and I love the feature. Recently I needed 4LO and it would only blink then go to 4HI. I took it to the dealer to check it out and he pulled some codes he had never seen before. After a call to GM tech support they determined that the after market remote start was the cause of the codes and the codes were what was keeping 4LO from functioning. They cleared all codes and the 4LO now works. I am wondering if anyone has heard of this before.

I haven't heard of that specifically, but I know that remote starts can sometimes cause issues with vehicles, even when someone else with the same model and year has one with no issues. I had to remove mine because I was having some difficulties with it too.