If your sales are to the general public ( who are not in a position to reclaim any VAT charged by you) then it generally does not make sense to register unless one exceeds the relevant threshold.
If you are operating as a sole trader then your PPS number (tax number) is used as your VAT number and is yours for ALL activities. So starting 4 different businesses as a sole trader, each with a turnover of say €25,000 and where each would be VATable businesses ( i.e. not exempt ) then you have a combined turnover from all businesses exceeding the threshold of €75,000 for sale of goods and you are liable to register for ALL.
VAT is probably the most complicated and tricky of taxes. It is very easy to go wrong in it and the penalties for going wrong can be severe. If you have any doubt as to how you should proceed you should take appropriately qualified advice.