What I meant was that there is no need to tilt vertical South-facing panels in winter.
There is no single solution for everybody. For the OP, in open spaces of Ca... If there is a room on the roof for that 3rd panel, why not do just that. With the 4th panel added as portable IF they decide they need it.
In a leafy shade they might want a portable to augment the total harvest, but then a single 100W won't add much and a couple of them will be twice as much of pre- and post- camping preparations. Also, in a partial shade I would go for parallel wiring.
When Santa Ana winds suddenly come blowing down canyons, the sound of portable panel falling on its face is loud enough to wake you up.