You can share the costs of the flight with your passengers as long as there's no element of profit to yourself. If you split it equally between the four of you, there's no problem. What you can split is the actual costs of the flight itself - including VAT and Landing Fees. If you own the aircraft, you can't factor in any fixed charges such as a proportion of the parking charges or whatever. If you're renting it, no problem, just split the final bill four ways.
Offshoreigor: the CAA used to have the same system - if any money or payment in kind changed hands it was considered Hire or Reward. A few years ago they realised this was too Draconian even for them so they changed the wording from "Hire or Reward" to "Valuable Consideration", thus allowing sharing of the actual costs of a flight between the passengers and Pilot, as long as the Pilot made no element of profit.
I've hunted through the CAA site for a reference, but unfortunately I can't find it, sorry.