ActiveMQ 5.x supports any Java platform of JDK 5.0 or later. To run on 1.4 see [these instructions]. You can use ActiveMQ 4.x on Java 1.4
ActiveMQ is based on J2EE 1.4 or later and so implements JMS 1.1 and a JCA 1.5 Resource Adaptor.
We regularly test ActiveMQ on Windows, OS X, Linux and Solaris though we're aware of folks who use AIX too. I'm sure we could certify ActiveMQ works on other platforms if required. If you have a platform that is not listed and would like us to certify it then contact our support team