The downside of this method is that the chicks are in the light all the time and this can sometimes encourage feather-pecking and make them slightly crazed, in the same way as humans exposed to twenty-four hours of light suffer a bit. They also cheep ALL the time, which can get on your nerves if you have them where you can hear them at night! However, if you are only trying out hatching before committing to buying a lot of kit, it is definitely worth considering. It can probably be rigged up from things that you already have - a desk lamp, for example - and the electricity will cost seven or eight pounds for six weeks.
The pad is taped (with gaffer tape) on to the bottom of a piece of hardboard that is propped up two to four inches above the floor covering of the brooder-box. The chicks can then tuck themselves underneath it as they would a hen, and come out to eat and drink. You gradually raise it up as they grow and they spend less and less time underneath. The pad warms the wood it is taped to as well, so as they get a bit bigger, they will start to sit on the top.
I usually screw the board to some two-by-four along two sides, but I have also had it propped on various combinations of tupperware or jam-jars - anything that won't collapse and squash them. Four inches is a bit high for day olds, so you need to make sure there is plenty of bedding in the box and the edges of the two-by-four are well down in the bedding. Once they get to a week or so old, you can raise it up a bit.
I like this method of brooding - it's more natural for the chicks as they are exposed to a normal day/night cycle, and it is MUCH cheaper than the traditional 250 watt dull-emitter bulb, which although very effective, costs about thirty pounds per six-week brooding period to run - which for small quantities of chicks is very expensive indeed.