I have been playing on a Bach 4G for about 3 years now. I love the sound that I get on the piece and it's a fairly versatile piece for switching between solo lit, band, and orchestra. However, I am doing a lot more principle playing this year as well as working on a hefty recital. I've been feeling a little fatigue and having to work harder the more I play. Since most of the playing I'm doing is in the mid to upper range at this point, should I make a switch to a 5G or something similar? Any suggestions?

My opinion: regardless of the mouthpiece your lips still have to make the same note. The vibration frequencies are still the same. If you have a good sound going I wouldn't suggest doing anything that might affect that. Some mouthpieces do help certain note speak better. Changing form the 4G to a 5G might cause you unintended and unexpected problems. MOst of my friends would pick between the two based on their intrument being used and not because of fatigue.

Things you said - -

A)
"I love the sound and it's versatile." (Two wonderful reasons not to change).

B)
"I am doing more principle playing ...."
"been feeling a little fatigue and having to work harder the more I play."
" doing mostly mid to upper range playing"

I suspect that you are practicing incorrectly as you find yourself playing more and playing in the higher range more often and for longer periods of time. I would be interested in what your practice schedule has been and what it is now. If you feel like posting it here feel free; or if you want to send me a message to my inbox I woul be happy to review your routine and make suggestions.