You won't be able to use all 4 axes at the same time. When you run the indexer, you park the carriage on the centerline of the rotary work piece. According to how your indexer is mounted, the X or Y axis is stationary. That axis then becomes the rotary axis A or B. So, if you had the indexer mounted along the X-axis, your Y-axis would be parked, then you would be moving X, Z, and A. You have to add a drive to your control box, so you will have 4 drives. The drives will all be energized, but the control software can only move 3 axes at a time. But, there is no physical swap of plugs or connections.
You can easily make that leg with the indexer. To make the off center slots, you wouldn't use a rotary cut file. You would just manually jog the rotary axis with the flat part of the leg facing up perpendicular to your cutter. Then just create a standard X, Y, Z flie from V-carve or Aspire to machine the slot.
I would go to the Vectric website to check on the capabilities of V-carve Pro with indexing. I use Aspire and I know that it would work.
PRT Alpha with 7.2 upgrade, indexer, and PC router