I’m a big fan of greed. Why play 3 colors when we can add more good cards with a 4th?
What adding colors will lose in consistency, it gains in power. Jeskai Midrange is a pretty sweet deck with some powerful threats.
Delver of Secrets is as efficient as they come, assuming you can reliably flip it, and it has seen lots of play in Vintage and Legacy as a result.
Young Pyromancer has also seen its share of Vintage and Legacy play as an excellent threat that can flood the board.
In order to maximize all of these cards, we’re going to need lots of cheap spells.
If we’re going to add a fourth color, we want to make sure that the spells we utilize aren’t necessarily going to be the earliest plays so that we can set up our Delvers, Young Pyros, Bolts, Paths, and counters early and often. But once we get past the first couple turns of the game, the value provided by Kolaghan’s Command is invaluable. It also combos extremely well with Snapcaster Mage recursion.
Tasigur, the Golden Fang is a great “top end” in that we have lots of cheap spells to delve away and it realistically costs only 1 mana, so we can leave up our Remands and Mana Leaks to play both ways!