I don't think you can. You can only get the SRU updates from the 'support' repo rather than the 'release' repo, so unless you have a support contract you only ever get updates if/when the 'release' repo is updated (which I think only ever happens when a new production point release comes out - 11.2, 11.3, etc). As I understand it, without a support contract the only way to test 11.4 is via a clean install.
Great. I downloaded the USB 11.4 Beta version and wanted to try the GUI. Well, it automatically started to install to my Solaris installation. I thought it would ask me if I really wanted to continue, but nope. It started to write over everything. I rebooted and have now lost my entire Solaris installation. Great.
Download the repository for 11.4 beta on oracle site.
Create a local repository on the machine in question (this will require couple of GB).
Using pkg command, set the publisher to point local repository.
Create a backup boot environment via beadm, which you will revert to if things do not work.
Even tho system update will probably create boot environment automatically i like this clean and named as i want
Try to update via pkg update --accept from the local repository.
This should update your system from 11.3 to 11.4
Often update from major to major release do state you need some SRU before, but i have tested several upgrades without issues between major releases.
Perhaps those are some borderline cases, but pkg update should fail in that case, leaving you with working environment due to the way solaris is patched.