It looks like an issue of web application configuration: for some reason the servlet resource provider does not recognize /servlet/ prefix as a name of local web application.

Did you place the v4 pd4ml*.jar in WEB-INF/lib or is it in the common lib/ directory? Since the new taglib JSP classes are in pd4ml*.jar, the location of the JAR (which determines which classloader is used) can be relevant.

For a smoother transition of JSPs from PD4ML v3 to v4, we recommend that you leave the JSPs as they are and store pd4ml_tl.jar from v3 in WEB-INF/lib. Generally, taglib v3 works well with PD4ML v4.