2 piece-bonus
+Scales with the Rapid Recovery talent without the duration being decreased.
+Stacks on-top of existing Renews beyond the 15s limit. Can go 60s+
4 piece-bonus
+Can trigger out of combat and even if the Renew is healing a full hp target.
+Has an internal cooldown of about 15-20s between procs. It almost always triggers as soon as the internal cooldown is up.
+Buff clears at raid boss start and the icd starts so won’t see a proc until 15-20s in.
+Seems to prioritize proccing when Renew is about to fall off.
+Instant procs via Holy Word Salvation as long as the internal cooldown is up.
+Does not reset Holy Words. You’ll need to reset them to use the charges so you might want to stagger your Holy Words just before the tier set procs.
Best practices
If you roll a Renew on yourself or others while in and out of combat you’ll almost always trigger the 4-piece bonus every 20-25s. If you’re moving around in dungeons leave a Renew on yourself and you’ll keep building up stacks.
In raid you’ll most likely have Renews out via Benediction or Holy Words, but hard casting Renews will help with the proc rate.
You’re not 100% guaranteed to get a proc though every 20-25s with just one Renew. The more the meerier.