Only four?
No way, and I mean No Way.

Cellphone. Be smart here and buy a high end smartphone. I've loaded compass, GPS and Swiss Army apps on mine. Very useful. Also make sure it has a torch function.

Key's. Two set's here. One to use and one lanyarded to you. Saves you a horrendous amount of grief if you loose/mislay a set

Cash money. How much is up to you. Take it as a given that the ATM always goes U/S at exactly the wrong moment.

Multitool. If it's a no-no for work, go work elsewere.

Torches. Always carry two. One to use, and one to not use. That way you always have a charged torch.

Lighter. The good old disposable Bic is first choice here. Again two. As above.

A small tin with basic med's, plasters etc is always useful.