First, make sure you have the capacity on the circuit to add another light. Then determine whether the supply goes to a ceiling rose /jb or to the switch. If there are neutrals in the switch, then simply take the live feed across to the COM of your fourth switch, and wire the t+e for the loft lights to L1, earth and the neutral. If the feed is at a rose or jb, or elsewhere, then take the feed straight to the loft light and drop a switchline cable to the new switch. Alternatively, take a new feed to the switch and then wire as in the first instance - probably not necessary to do this.
You probably have to widen the back box too, but that will be obvious. It's possible with this set up to leave the loft light on without realising. If you want to take the 4 gang switch back to the shop, you could fit a new 1 gang switch in the loft. This would be much easier a) to find a feed (from a ceiling rose or other supply) b) to wire the switch c) to avoid leaving the light on.
