iReport 4.0.1 requires JDK 1.5 or 1.6 . It is notoriously incompatible with Java 8 or higher.
Released in 2011, version 4.0.1 introduced several critical stability fixes and expanded data source support: jaspersoft ireport designer 4.0.1 download
To ensure the designer runs correctly on modern systems, follow these requirements: iReport 4