Excel 4.0 macros are a 30-year-old feature of Microsoft that has been gaining popularity among malware authors over the last year. This type of macro code is currently being weaponized by threats to deliver additional, more persistent malware.
Similar to the more modern VBA macros, this technique is effective because Excel 4.0 macros are a component of legitimate Excel functionality. Their misuse will likely continue since they won't be disabled, are used regularly for benign business purposes.
Read more about the Macrosheet Evolution!