Is there such thing as too much green tea? That's pretty much all I drink. Usually 4-5 mugs a day.

Yes. As the song goes, "Too much fun whats that mean? It's like too much money, there's no such thing. It's like a girl too pretty,..." But for everything else in life, do in moderation. Plain green tea at 4 mugs of 12 ounces each day sounds ok. That means without creamers, sugars, or other sweeteners... and without tapioca pearls (boba). Drinking plain green tea is better than drinking sodas.
Yes. It is not good to drink so much tea despite the hype. Tea has a diuretic effect. You should drink more water than tea. Have a balanced diet and approach to lifestyle.