You can buy a subscription for Strawberry Prolog which includes the current version and all future versions including 4.0 . When you bought a subscription you will receive a personal password which can be used to download all future versions including 4.0 . The cost for this subscription for the moment is only $249 USD for the Professional edition and $98 USD for the Middle edition. Also you can use the student discount which we offer.
If you want to buy a subscription for Strawberry Prolog , please send a check or money order to the author . or pay directly by your credit card in our download center (dobrev.com/passwords.html ). When we receive your order in 24 hours we will send back to you an e-mail with your personal password and later we will send to you a letter with your license. You will need the password to copy from our web page the versions 2.x, 3.x professional (or middle) editions and the version 4.0.