build(sbt): bump max heap to 4g

PortalTest and AdvancedSearchPageTest are both flaking. They used to be
fine, and due to the old setup with the `.sbtopts` file the max heap was
actually always at 4G so might as well try returning to that.

.jvmopts

@@ -1,4 +1,4 @@
 -Xms2g
--Xmx3g
+-Xmx4g
 -XX:ReservedCodeCacheSize=256m
 -XX:MaxMetaspaceSize=512m