Rob,
GEP 10.2.1 ships with httpclient 4.2. If you need an older version, you'll need to drop it's jar files into GEP's deploy folder. This may become an iterative process as the older jar files may also require other jar files that aren't available with GEP 10.2.1--so you may have to drop its dependencies into the the deploy folder as well.
To summarize:
1) Build your processor using the 4.0 dependencies in your pom.xml file.
2) Drop your custom jar along with httpclient 4.0 (and dependencies) into GEP's deploy folder.
Hopefully this works, if it doesn't there are ways of build jar files that contain all of their dependencies you can use.
-Javier