Reading the numbers
A pair of sixes can be dealt 6 ways and a pair of fours 6 ways. Once both hands are out, the cards they share leave 36 possible pairings. Each row above stands for every pairing with the same suit pattern. The best case for 66 is 82.1%, the worst 80.7%: a shared suit takes away flush cards, and a suit held by both hands can split a flush.
Equity counts half of each tie, so it is the share of the pot 66 takes on average if the money goes in now. Compare it with the from the pot odds calculator. For a whole range on each side, use the range equity calculator.