4 links are the bars infront of the axle, then the 5th link can be a Panhard rod or Watts linkage (as above sometimes called a 6 link kit cos it does link to the chassis at 6 places).

If it is prepped for 5 link then you will need an atlas axle with the mountings, a ZF diff, uprated half shafts etc. You will also need the link bars and bushes and the actual panhard rod itself which runs across the back of the axle.

Rally And Competition Equipment (RACE) in Stoke will be able to sort you out, but a decent axle aint cheap at all. Brand new their options range from 3.5k upto over 5k! And most of the diff options are of a short ratio rally variety.