Will this arrangement of bricks be a good base for a concrete floor for a garden shed?
I’m no expert but it looks like you’ve made a good job for your shed base and will be more than substantial for a 6x4 shed. When you talk about ‘cement’ you mean concrete? So that’s cement with aggregate in for strength. If the wooden surround is level from side to side, once filled you’ll be able to strickle across to produce a smooth and level surface (or sloping fractionally for drainage?).
Regardless of the underling surface the shed should go onto a slight wooden frame (use tanilised wood) to raise it slightly to avoid the base of the shed from rotting. Once your concrete base is finished and the surrounding ground levelled (or whatever you intend doing with it), see how the water drains and this will determine the height the shed needs to be raised. Usually this is only 2-3cm. Good Luck!