As stated in the spec :
The minimum required version of the HTTP specification that a container must implement is HTTP/1.0.
It is strongly suggested that containers implement the HTTP/1.1 specification as well.
There will be restrictions for containers supporting only HTTP/1.0.
For example :
In containers that support only HTTP/1.0, only the doGet, doHead and doPost methods are supported, as HTTP/1.0 does not define the PUT, DELETE, OPTIONS, and TRACE methods.
Tomcat4 does support HTTP/1.1.