It is planned to move to Java 8 early 2015 currently (might change depending on GDA requirements). Java8 allows the isosurface feature to be added to DAWN (existing already). It also allows some speed up by using parallel streams with lambda expressions.
e4 is being developed with gtk3. There are some existing GUI bugs with gtk2 which e3 has fixed and e4 will not address - for instance the truncated toolbar bug.
RHEL7 comes with gtk3 and RHEL6 comes with gtk2 therefore Diamond Light Source should not move to e4 as the target until it moves to RHEL7. Once Diamond moves to RHEL7, DAWN and GDA client should use the compatibility layer with e4 to run its e3 code and start to migrate gradually to e4. This will take some time. Controls are already using e4 with gtk2 and have not mentioned the truncated toolbar defect as yet (will tell them...) however previous user feedback as said that this is a show stopper because actions required (for instance plotting tools) disappear from the toolbar.
This is probably more attractive to move to than changing the UI to e4 because there are no defects encountered when doing this in RHEL6 since windowing tool-kit does not matter. Therefore DAWN will continue to provide its code in a service oriented architecture such that these services may be reused on a future OSGI based GDA server. For instance, expression service, conversion service, image processing, (many others:)