About 50 years ago an engineer at TI invented a 4 quadrant sensor, used for guided missles.
Same concept
up/down difference = (sum A + B) - (sum C + D)
left/right difference = (sum A + C) - (sum B + D)
You need 4 light sensors
you could do the whole thing in analog electronics, op-amps and such,
OR
do it all in software using 4 A/D channels on a processor, like an old robot controller.