Am looking to go from Solarus Update 4 to Solaris Update 6.
I've downloaded the U6 patch cluster and installed, but it seems to me this ins't a true upgrade. Am I wrong? Is there a better way to upgrade my system?
A recommended patchset does what it implies, apply patches to close security holes, enhance O/S features, etc for a specific version of the O/S.
To update from 10u4 to 10u6 I would obtain a 10u6 distribution DVD and boot from it. As it enters the install routine it will check for the presence of any previous version of Solaris and offer an "upgrade".
NOTE: Should it fail to offer an upgrade you would NOT allow the routine to proceed to a new install thereby overwriting your O/S and do a post-mortem as to why it didn't offer an upgrade.
After such an upgrade you would then apply a recommended patchset if needed (depending on what the release notes say that the patchset fixes that you think should be fixed).
Beware that 10u6 is seven years old so that would be a very relative upgrade.
In any case, make sure you have a fully operational and tested backup/restore procedure to revert the system as it was before the upgrade should anything goes wrong.
In addition, having live upgrade configured first would simplify and secure the upgrade process, minimize the downtime and ease futures upgrades.