raising level to 4 inch on existing concrete base

I'm just a plasterer and really you want to speak to a bricklayer, but im used to render and coverage upto 1 inch depth, lets say your shed base is 15 sq mtr, from my experience 1 bag of 25kg sand alone will nearly do 1 sq metre 1 inch depth, but you have 15 x 4 sq mtr. 4 inch depth means you need about 100 kgs of material per square mtr so thats 1500 kg of material, getting bulk delivery of pre mix concrete or bulk delivery of all loose materials in one delivery but look at or work out the price per kg on sand, loose price is far cheaper than the 25kg bags, if you can then have a ton of sand delivered with 300 kg other material and 200kg of cement depending on your base mix, from a supplier who sells sand and aggregate loose, hope this helps

It may be more cost effective to shutter the existing base then get a barrow mix company in. They will barrow it in, all you do is level it and float it off. You should be looking at around 1.5 cubic meters and the barrowmix firm only charges for what you use.