How do I give Nexus more than 4Gb of heap memory?

There is a 4Gb limit imposed by the Java Service Wrapper watchdog process used by Nexus.

If you need more memory, comment out the wrapper.java.maxmemory line in <nexus_root>/bin/jsw/conf/wrapper.conf and add a line to the wrapper.java.additional arguments. Example:

wrapper.java.additional.4=-Xmx5120m

Make sure the sequence number (shown here as "4") is consecutive with the other additional argument lines.

Note: It is very unlikely that you actually need more than 4Gb of heap space.