If you prefer to stay on version 4, you will be welcome to do so. There will be no obligation to upgrade. We will continue to support version 4 as long as it is viable to do so. Only when the vast majority of merchants have upgraded to version 5 will we consider ceasing to support version 4.
Considering the massive leap in functionality that version 5 represents, it is worth upgrading as soon as possible. Although the fee structure has changed, ezimerchant version 5 will deliver so many time savings and additional services that the total cost when compared to version 4 will be much lower in real terms even if you end up paying us a little more.