You're *sure* you need another roost? 4' should be plenty space for 5 chickens.
The first thing that comes to mind would be running a 6' roost (with droppings board under it) lengthwise in front of the rear wall of the coop, with the droppings board maybe 2' above the coop floor and the roost maybe 1' above the droppings board. However if you did this, a) you would have to re-rig your waterer and feeder and might well start getting a bunch of shavings/poo kicked into them, and b) you would have to put in some sort of stepstool kind of arrangement to help them get up to the roost.
I might be tempted instead to make an L-shaped roost, running where the current one is now (but maybe raised up with a droppings board under it, as per previous paragraph) and just adding another arm going off towards the right-hand wall of the coop. You *could* put in a leg to support it where the two parts join but if solidly and sensibly constructed you may well be able to get away without, for just 5 chickens. You'd have to wait and see whether you run into problems with the waterer and feeder that way - depends on whether the chickens tend to try to jump down off the roost that direction, or not. But if necessary you could always put in a wire mesh 'half wall' going down from the ceiling, to the L of the central roost, to prevent them from disembarking thataway. Alternatively you could abandon the droppings board idea (although they are terrifically handy, IMHO) and just keep the roosts at ground level like now, which would mean just adding the L extension the same way you constructed the existing roost.
I still think 4' should be enough room for 5 chickens, though.
BTW if you haven't already (and you may have), you'll want to raise the waterer and feeder up so they're at the level of the chickens' backs i.e. they do not bend over at all to drink or eat. This will greatly reduce the amount of floor crud that gets kicked into them.
Cute coop
Good luck and have fun,
Pat