you could order the windows with the nailing flange. prep the opening with Grace/vector or equal install your window ( I hope its smaller then 42")
then you would install some sort of flat trim 1x on top of the nailing flange. I like to use a PVC composite trim like AZTEC they even have a pre-made window sill. install the sill and then the sides and top trim remembering to install a drip cap at the top of the window trim. important ! pay attention to the correct way to flash the window.
or you can install a 3/4 parting strip on the sides of the 4x4 install your units with out the flange into the opening. press the unit into caulking that you will apply to the parting strip. secure the unit with screws and then bend some alum trim coil to cover the outside 4x4 and rest.