I'm also using a T4 with a XP8 and I find that the range is worse then it would be with a T2C or T3. Not sure why this is but this isn't the first time I've experienced it. I just played with the position of the RM433 and added another one....it seemed to take care of it.
Did you also try turning up the IR ports? Or is the RM433 just not receiving a signal at all?

I also think it's about time the T4 came with "N" or at LEAST "G" wireless. "B" is dead slow!!!