Fitting new dimmer to old wiring but I've got four wires and an earth
The instructions for that dimmer switch assume that you only have a "switch loop" wired to the switch.
A "switch loop" brings an always-on Live wire down to the switch and carries the switched Live back up to the light fixture.
In this case, the always-on Live should be Red and the switched Live return should be Black.
The Neutral side of the power connection stays up at the light and is not brought down to the switch.






But what you have there is the always-on Live and its associated Neutral both brought to the switch, and then the switched Live and Neutral sent up to the light.
The Red wires are the Lives and the Black wires are the Neutrals.
There's no way to know for sure just by looking at your picture which of the Reds is always-on and which runs up to the light.
The switched Live should be marked somehow and it's possible that the red tape wrapped around one of the red wires is intended to be that mark.
But in your case it doesn't really matter because your dimmer doesn't care.  

Just disconnect the 2 red wires from the old switch and connect them to the 2 terminals on your new dimmer which the instructions tell you to use (C and L1).

Also, as RedGrittyBrick pointed out, your new dimmer has an Earth terminal which your old switch did not.
This terminal must be connected to the green Earth wire in the box using a wire with green/yellow insulation.