You mean LIPO?
Yes, if so, but you should be able to get some dimensions to fit the Tray in the B4.
To get BRCA legal ones would be my best advice: See
BRCA Website for the Electric Board lists and legal Lipo Packs. You will want sticks if you run a B4 only, consider some saddle packs if you want to go down the route of 4wd as well in the future.
Please be aware that you also need a charger capable of charging lithium (and hence LIPO) batteries. They use a charge method known as CC-CV (Constant Charge, Constant Voltage). Some clubs also only allow charging in a lipo bag/pouch.
The chargers can be had quite cheaply (approx £30-40). Recommend a good UK RC Racing distributor (MK, DMS, Microtech, Apex, Raceplace are just a few to name who have a presence on here).
Hope this helps.