IzPack 3.11.0 uses methods from Java 1.5 and as such its installers won't run a Java 1.4 as currently stated in its Introduction
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Ideally, you would document when this break ocurred such that users can decide which IzPack release is the last to choose if Java 1.4 is required to support.
Yes, IzPack will officially be 1.5-only. Sadly, we lost track of when we accidentally became 1.5 only...