I would definitly get the 4 set regardless of the mode. Even on 10 man there are several fights where the 5% increase is much better than the small loss in stats.
However I would reccomend also keeping the other items for fights (talking mostly 10 mans here) where HR does not hold a huge percentage of healing. It is not hard to swap between the 2 gear sets depending on fight.
As for Ultraxion. You should be 2 healing it the first few times and then solo healing from then on. When 2 healing it I would get Red and Blue and our Shaman would get green.
Now I am solo healing the fight I just grab Red and the boss dies so fast the blue buff doesn't even spawn anymore.
Alot of Raid Leaders will mistakingly think "We need 3 healers because the boss does epic damage at the end and we need 3 healers to cover it." They do not think "well maybe if we use 2 or even 1 healers we could probably kill the boss before he reaches the epic damage phase that needed more healers." You should always be trying to go for the least amount of healers as possible.