Most of the code in the web service client is generated by Axis from the Alfresco WSDL, and since Axis supports JDK 1.4 there's a good chance you could simply recompile that code with a 1.4 compiler without incident. That said, there are a number of hand-written classes in that API as well (the ISO9075 class, the WebServiceFactory etc.), and those may have some 1.5 constructs in them - if so it shouldn't be hard to copy that code into your own package and modify it to be 1.4 compatible.
Of course you could always just work from first principles (the Alfresco WSDL) and roll your own Web Service client from scratch - because there's very little hand-written code in the Web Service client this would be quite practical. This also has the benefit of allowing you to pick which SOAP framework to use on the client side - you aren't forced to use Axis.
Cheers,
Peter