Should we get the 2-piece bonus? Yes. In raid, you can expect 2-4% extra healing on most fights, and maybe even higher depending on your playstyle. Also, the 2-piece is a significant buff in dungeons, since Healing Surge is your primary heal (see Dungeon Healing Guide).
Should we get the 4-piece bonus? Yes. In raid, you can expect between 4%-7% extra healing on most fights, depending on the HPS requirements, your playstyle, and whether or not you have Praetorian’s Tidecallers. It is significant.
Does the 4-piece proc when you generate a Tidal Waves charge or when you spend one? It procs when you generate one. That’s why it’s so powerful. Frequent chain heals are still potent with the 4-piece. (Note: it only procs −3 seconds when you Riptide with Crashing Waves - it doesn’t count the two charges as −6 sec.)
Which pieces should we get? Llenne has curated a community BiS guide for Nighthold that gives some starting points.
Is it worth dropping Drape of Shame for the tier cloak? Probably, if you can get a higher item level one. As one of the tier pieces with high mastery, the tier cloak is valuable.
I don’t like those stats. Should I take the 4-piece? Yes.