You will need a resistor, but as 4s is 14.8v you can use a 680 ohm resistor. You can also just use some prewired leds for 12v, they will be fine at £15v - i used them before. https://www.ebay.co.uk/itm/3mm-5mm-D...r=502685164120 - just some i found on ebay. there are plenty of sellers. Also you can use 12v led strips - i have used these on 5s.