One can also use the content-aware scaling tool in the vertical direction to increase the height sufficiently for the aspect ratio to be exactly 4x6 without causing too many problems.
I used content aware scaling to quickly add some length to their legs as well as increase the vertical dimension of the sound diffuser grill (the vertical slats L and R of their heads. While hardly any problems are caused by increasing the previous two heights, you don't want their heads to become elongated, so a bit of masking and filling in the background is necessary (...BTW, I pretty much ignored this aspect to save time for the little demo posted below...). If you like the overall look, people can walk you through the various ways to create a better background. particularly in the area of the vertical sound diffuser slats. This example is exactly 400 pixels by 600 pixels.
Tom M
PS - In contrast to Larry, I prefer a darker background so that the subjects stand out better, and made it that way so the OP can decide which he prefers.