Hi Ralph... for your 8m x 4m driveway, you will need 32m2, but not just of tarmac....
A tarmac surface comprises different materials laid on top of each other, the thickness of each depends on what the surface is going to be used for - for example a motorway would need to be much stronger than a footpath.
Do you have a tarmac driveway already which you are just going to be renewing ? If so, you probably already have most of the materials in place, you just need to replace the upper layer(s). If not, then after excavating down to the correct level and smoothing everything out, you will need to put down a permeable geotextile membrane, then a layer of *sub-base* (roadstone), correctly compacted. A *basecourse* of tarmac with larger aggregate (say 20mm) goes on next, again properly compacted, followed by a *wearing course* of tarmac with smaller sized aggregate, again properly compacted and rolled to give a smooth finish.
You may need to also put in some sort of paving around the driveway to contain the tarmac materials, these are usually bedded on and backed with concrete.
I would recommend that you contact a professional company who can design the tarmac driveway for your needs and will ensure that the correct thicknesses are used and that it is laid properly. If it's laid badly or of inadequate materials, it will look awful and soon deteriorate.