I have a pro licence which is apparently limited to 2 licences and I want to upgrade it to 4 licences, but I cannot find our from your website how to do this or how much it would cost
You need to purchase the additional licenses using the same email address as you used for the first two. Two licenses purchased together will cost you $64.95. Purchased individually the will cost $49.95 each.
You can do that at http://www.emclient.com/purchase-em-client
If you have any further questions about purchasing or adding licenses, it might be quicker to contact [email protected]