I understand your need for a solution but this issue isn't an incompatibility between SmithCart and DNN 5 or 4, it is an issue related to the specific configuration of your Cart when using PayPal Standard which was resolved in a later version and listed in the Change Log found Here:
http://www.smith-consulting.com/Produ...
Every release of the cart has at least a couple bug fixes which is the nature of business for any software company and issues that have already been resolved in current builds cannot be easily patched in older versions and may require an upgrade to the latest build. Our support department cannot provide custom bug fixes for obsolete builds or customized deployments. If we used the logic of providing custom fixes for free when a bug was found in an older release then we would essentially be fixing all past releases of the module for free for every customer for a lifetime.
The cart comes with 3 months of free upgrades from the date of purchase which is in line with other module vendors. We ask that customers maintain an active cart maintenance plan so you will have access to cart downloads area throughout the year as needed and we release new versions of the cart frequently with bug fixes, security patches and new features which provides a level of protection for your ecommerce site.
In addition to upgrading your Cart to resolve this issue, it is also strongly recommended that you upgrade your DNN instance as there are certain hacker groups that are actively exploiting security vulnerabilities in old versions of CMS platforms such as websites running old versions of DotNetNuke.
Once a vulnerable website is compromised, it is then leveraged to launch network-based denial-of-service attacks. This causes significant grief for both website owners and hosting providers.
It is your responsibility to maintain the security of your website.
If you are running an old DotNetNuke 3.x/4.x/5.x website:
* Your website is vulnerable and may have already been compromised.
* Your website software is extremely old.
* You need to upgrade your site to a secure version as soon as possible
Hope this is helpful!