Prefrabicated workshop base

You can leave out the hardcore...the only time this is required is if it's got things growing in it, IE if it's now a flowerbed, if it's lawn, simply remove 50mm of lawn and topsoil and concrete over, putting 100mm down so that it's raised a few inches up...if it's already hardstanding (gravel, concrete, tarmac or a mixture of all 3), simply put up shuttering and phone readymix.

I want to lower the base so I have had to remove to tarmac that was there, and have dug down into the soil. If I compact it with a wacker, can I just layer the 4in concrete on top of that?

There's no hard and fast rules, and 4in of concrete is plenty deep enough, but I would suggest having the floor a few inches up from the surrounding area to prevent flooding/water ingress.

If the ground below the tarmac is soft, then whack it, if it's firm underfoot, then it doesn't need whacking, but it's up to you, a whacker is around £15 per half day, and some steel mesh is around the same for a 8 X 4 sheet...there's no limits on how far you can go, but at the end of the day, it's a shed - I've seen them erected on a patch of 2 X 2 flags and it was OK.

make sure you put a sheet of plastic at the bottom of the base, it makes it much easier to work the concrete and prevents moisture penetration.

Lawrence Zarb pretended :

You should try not to disturb the final surface of the soil upon which you are laying the concrete. Disturbed soil is less dense than soil which has lain undisturbed and will compact down under the concrete.

Phil,

I actually have my own wacker, so wacking the site is a zero cost option