OK, we have spoiler tags, so I'll give my opinions inside spoilers...
These problems cover 2 points:
1. Is each hand worth adjusting to 20?
2. Under what circumstances might you change your mind?
Remember the 4 situations:
a) the score is basically even, early on in the game (so score doesn't push you to be aggressive OR to be conservative)
b) your side has a big lead (420-150, say)
c) the opponents need 31 a couple hands in a row to have a chance (480-200, say)
d) double bidder out (450-450 or the like)
Why did I pick these? a) is the generic case, that holds most of the time. In b) and c), my principle is this: DO NOT go set. PERIOD. DO NOT shave requirements. In d), the exact opposite is true: aggression is demanded.
So...
Hand 1
a and d. Pass in b and c. You have no tricks; partner is going to have to carry the burden by himself. And he hasn't shown interest in doing so. I *do not* want to encourage him to bid with a mediocre hand.
Hand 2
Always give 20. This hand is 14 with diamonds trump...look at your trump support in that case! Or it's 16 (or 27), with aces to make giving 20 easy. So no matter what the trump suit is going to be, I've got a very decent hand for partner. Ergo: upgrade, even in conservative situations.
Hand 3
This hand is about recognizing the aces you probably can't count, for adjusted meld, as is hand 4. First: partner's trump suit is probably NOT going to be clubs or diamonds; he's much more likely to have heart or spade length. You have at least 2 vulnerable aces...the 3rd club ace, and the spade ace. Conversely, tho, you do block communications between LHO and RHO, so there's less chance they'll both cash before you do.
I would always give 20 in d. I would probably NOT give 20 in c...I'll take my chances that we can pull 20 on defense. I won't give 20 in b, unless I think my partner knows what he's doing. The typical "cash all my aces then lead trumps" player would make this too risky. Worse, the player who starts by playing HIS trump aces from the top, would be much too risky...spades is the most likely trump suit. I'd probably give 20 in a...with an apology ready if it doesn't work out.

Hand 4
As per hand 3...this is about recognizing that these aces are NOT all cashing, most of the time. Assume hearts or spades will be the trump suit...because much more often than not, that will be the case. Here, everything suggests pessimism. You've got 3rd aces in long-ish suits...the risk of someone having a doubleton (in at least one of the suits) is fairly high. You have 2 open suits that give a clear path from RHO to LHO...and with terrible lengths there.
So I can't really see this as a 6-trick hand by any stretch. 3-4 tricks, probably. Another disadvantage: no marriage, but with those 6 aces. Partner may have a hand with below-average strength...2 aces, say...and be afraid to bid. OOPS...we're set. So, for the most part, I'm *passing* all the time.