I have an image of a cake:
I'd like to split it into 5 cake slices (ie from the centre) as I want a slice per A4 page
Is this possible in photoshop? or easy to do in any other free/online software?
Yes, it's quite easy to achieve using the Polygonal Lasso tool in Photoshop. (You may wish to delete the background first using the Magnetic Lasso tool.)
Use the Polygonal Lasso tool to select the first pie slice then cut it to the clipboard. Then create a new document and paste the slice on to that.
By cutting and pasting in this way you are moving the image pixels so you avoid the risk of accidentally copying the same pixels (on the edge of a slice) to more than one of the new A4 images you are creating.
Tip: You will probably want to select Antialias on the Polygonal Lasso tool and use a small feather (1 px) or none at all (0 px).
If you want to do it in accuracy, you can,
Create a new layer, draw two diagonal lines, one from top left to bottom right, another from top right to bottom left, then you get a center point.
Using swirl gradient tool to make a swirl centered at the center point, from white to black.
Convert this layer to indexed mode, with just 5 colors. And convert back to RGB mode, do some Gaussian blurs if you feel the edges too sharp.
Then using Color Range tool, to select for each distinct color, apply the selection on the background layer (i.e. Your cake image layer), cut the selection and paste to a new A4-sized file.
I can't tell more because I didn't have Photoshop installed in my Linux box. but you can figure it out in details.
You have now the original picture, with 5 different selection that you can use to select every piece (and cut&paste).