Section 5.1.2 defines the bundling rules for AS4. It states that a Receipt signal MAY be bundled with any other message unit.
This suggests that other message bundling options are not allowed. In section 2.1.1 - Message Packaging however support for more general bundling is required. Also section 2.2.1 describes bundling of Error signals and User Messages.

It is unclear which bundling options must be supported.