Use four individual subarrays, resonant-series, fed, with pin-type feeds.
These subarrays can be individually constructed, which addresses limitations
in mill bed size. Mount these subarrays onto an FR-4 substrate that
has a large square slot cut in the middle of it. The feed pins from
the subarrays will be in the slot area. In this slotted area will
be mounted the beamforming/TR board (maybe LNAmixer too?), directly in
contact with the four subarray boards. This board will use the outputs
from the four subarrays to generate traditional monopulse outputs, then
switch them into the receiver. The sum channel will be switchable
between the transmitter and receiver.