Is 2.8 the rated current of the motors?
Do you know what current you can nominally run them at?
How many drivers are you currently using on the Maestro now?
Would adding the 2 driver expansion board give you enough drivers if the internal maestro drivers are able to power the extruder motors sufficiently? The onboard drivers can supply 1.6a max with cooling. If that's not sufficient you would need to use external drivers that accept step and direction signals. There's also the problem that the Maestro is no longer produced so it may be difficult to find the expansion board now.
https://duet3d.dozuki.com/Wiki/Dual_Stepper_Driver_Expansion_Module
Given all that, I think the best way to drive 4 2.8A extruders with a Maestro is to replace it with a Duet3 6HC and 3HC expansion board depending on how many total drivers you need.