If I have 20th Level, fully traited mythic - right now that troop by itself in a troop lineup is valued at 3995.
Yet when I add 4 20th level fully traited mythics, each by themselves are 3995, how come when they are together they add up to much less than 4 x 3995?
Because Team Power isn’t only about the troops in the team. Kingdoms at level 10 give 40 points each, and your mana masteries and guild masteries play a part as well. So your initial 3995 isn’t only the troop’s value, it’s the troop’s plus all your player bonuses as well., and they apply to the team rather than to individual cards.
The actual value of a level 20 fully mythic troop is 1275. Which is how much it should increase by for each copy of that troop you add to the team.
EDIT: Guild bonuses also add to team power at 40 points per skill point bonus I believe.
do kingdom stat bonuses apply? or just the kingdom levels? how about kingdom power?
Kingdom stat bonuses are the only thing that apply. When you get a kingdom to level 10 you get the 40 points for the +1 stat. It goes up another 40 for the bonus increase at 3 stars as well I believe. So in short, kingdom levels and power only affect it in so far as they give stat at certain points.
EDIT: I used to have a working calculator for team power, but the guild update messed with the formula for guild masteries contribution to team power, and I don’t have enough data to fix it at the moment.