It costs about $3/real watt to build a top quality fixture. You might get away with $2/watt but hard to imagine a lot less for a good light. You will need the following:
LEDs, heatsink, probably a fan unless you buy a great heatsink and that's money too, driver (could be dimmable and then may need to buy a potentiometer), power cord/plug for the driver (if not included), maybe a power supply for the fan (unless included), thermal mounting pads or thermal grease and/or thermal adhesive, hangers, wire, solder.
You have 4 x 4ft = 16ft2 of space to cover. Let's say 40W/ft2 and you want something like 16 x 40 = 640 watts 640W x $3/W = $1920 or at the shallower end of the pool 640W x $2/W = $1280 Now the reality is that you could get away with fewer watts with the better fixtures because you are creating more lumens/watt with the higher end components so we'll readjust a little on the estimate and say you need 500 watts of excellent fixture so 500W x $3/W = $1500 These are obviously just rough numbers but reasonably close to reality for growing quality flower.
Supplemental LED fixture - you have 48 inches across so build the fixture about 40 - 44 inches long on an - or couple - of aluminum tube heat sinks (see stevesleds.com) with a pressurizing fan, good driver (inventronics, meanwell, maybe steve's drivers?), thermal mounting pads, mounting hardware for fixture. I'm just going to guess that you could easily get an excellent 150W fixture and you'll be about a third or so of the way to a quality all LED grow....and have an excellent supplemental light to start.
stevesleds.com ledgroupbuy.com heatsinkusa.com modularled.com digikey.com mouser.com