Right, so if you run a 4awg cable its not likely to fit the amplifer terminals then.
If JBL recommend a 8awg cable then the first, cheaper, kit will do the job. But it is at the top of the 'performance envelope' for the kit you've specified (600 watt)
Personally speaking, I only like doing something once; on that basis, if it were me, I would run 4 or even 2awg power and earth wires and sling in a distribution block. From the distribution block you can run an 8awg cable to the amp you have now (and seperately fuse it) and it still leaves you with capacity for adding more kit/sucking more power through the power cable you've installed.
If you upgrade that JBL amp or want to add any more power-hungry kit, you've got to start again right from the battery.
Personal choice dude. If you can spare the extra £30-£40 it'll cost to go up in cables and add a disty block, I'd do it without thinking.