Is there a guide to updating to V4 from V3?
 
Have downloaded the V4 version but docs only specify new install?
 
cheers

Well actually there is available an article about how to do it:
http://www.viart.com/upgrade_viart_shop_3.7.html
 
Be aware that if you have your own table-based custom design there will be a problem after you upgrade - the design will need to be converted to the div structure.