help with massaction

-- HikaShop version -- : 2.2.2
-- Joomla version -- : 2.5
I need to update the characteristics variant prices for mass products.
I have only a single price per variant at present
I want to add that if 5 or more of a variant are bought the price is £x
I have 4 variants per product and each variant will have a different price so I can't use a discount.
How would I do this.
All help appreciated.

thanks