I personally would hlbc before you start doing lvl 4. Also build a bunch of lvl 1 hoard frost (around 15-20) and upgrade those before going back and buying lvl 4 hl buildings. The hf lvl 2 cost the same as lvl 4 hl but give better stats.
Was clear enough to me. And no wasted space with uneeded filler on a fairly straightforward question.
I find that max Titans lair give more plunder than a lvl 3 torture garden. A lvl 4 torture garden gives more plunder obviously. So I don't change them unless I have the cash to fully max out. And a level 2 hf gives more plunder than both.