Relative amounts of luck in MTG limited formats?
You pretty much answered this question already. While I haven't heard of 4-pack 30 card sealed yet,I would say this is the format that is the most dependent on luck. A single bomb in a 30 card deck has a much larger impact than in a 40 card deck, for 2 reasons:

1) With 4 packs, opponents have a lower chance to equalize a single bomb of yours with at least 2 solid rares of their own, simply because they only get 4 rares instead of 6.

2) Assuming the average game runs for at most 15 turns, you get to see 22 cards of your deck, ignoring cantrips and library/graveyard manipulation: That is ~75% of a 30-card deck, and only ~55% of a 40-card deck. So you have a chance of up to 75% vs. 55% to draw your bomb in a game. Or stated differently, to get your bomb with a 50% chance in any given game, you need to draw only 15 vs. 20 cards, again ignoring library/graveyard shenanigans.

Obviously, this applies to every participant, so it all averages out in the long run, and it comes down to skill again. But in the 4-pack format, I would expect the standard deviation in deck strength to be significantly higher than for 6 packs, or, to answer your question, the luck factor is highest in the 4-pack, 30-card deck sealed format.