Excuse my ignorance, but I sometimes see a build type referred to as "rebel control". Is this an archtype in the current meta, or even the upcoming one (wave 5)?

I believe it is a Rebel squadron that utilizes a number of ships with some kind of Ion Weapon. The most basic form would be four Y-Wings with Ion Turrets although you could see some B-Wings or HWKs thrown in as well.

Ion Cannons and Ion Turrets in combination with "Stress Cannons"

Stress Cannon being Flechette + Munitions Failsafe and if on B-Wings and E-Wing "+ Accuracy Corrector"

Or any X, Y, E with R3-A2

A ship Ion + Stress once in effect is very difficult to get out of unless the ship has an R2-Astromech or Nien Numb. Both of which are Rebel specific.

Beings withthr crew who gives out stress at range 2 and ion cannons can do both with a range depending.

The name 'Rebel Control' is given to an archetype of list that attempts to use maneuver restricting, and action denial techniques, as well as positioning to exert a larger than average amount of impact on the opponents available combinations of choices. Lists of this type will generally use 'weaker' utility pilots and upgrades to accomplish this task, foregoing some health and power output to attempt to have a much higher control of the gameplay itself. While many abilities and lists will utilize these tools in some ways, a 'X Control' list makes it the center of it's development. The term comes from collectible/trading card gaming were 'xyz Control' is a deck archetype. (e.g. Blue Control, America Control, Esper Control in Magic:TG, or from what I'm made to understand Tau Control and Eldar Control in WH40K Conquest)