When I put my truck in 4HI the green light blinks instead of staying solid green
green light on 4HI on 2004 Toyota Tundra
This means that some aspect in your 4wd system is not functioning correctly, I would have the codes pulled from your transmission and power train computer to see what is not working properly.
I have the same problem when I switch to 4Hi while driving. If you stop or slow down below 20 kph, it should engage the 4Hi and stop flashing. The flashing means it is trying to engage the 4Hi. I've also had issues where it stopped flashing and was solid green, but was definitely not in 4Hi. It's been getting worse in the last 2 years. Will have to get it checked out.