Minecraft – How to see the values of the enchantments in Minecraft

Yes. According to the Minecraft Enchantment Calculator, there is a 9.5% chance of getting 4 enchantments when using 30 levels on a Diamond Sword. It also doesn't break any enchantability rules. You just got really lucky.

Yes, but with rapidly diminishing returns

According to the wiki page on Enchanting¹, the effect of Unbreaking is:

Increases durability

[...]

For armor, (60 + (40/(Level+1)))% chance a use reduces durability. (In other words, each durability hit against “unbreaking” armor has a 20%/27%/30% chance of being ignored.) Thus, on average, armor lasts 25%/36%/43% longer.

You can see that the formula is different for armor, and even with the regular three levels, the chance to not lose durability when hit increases slower with higher levels. Extrapolating the "20%/27%/30%" using the formula given yields:

Unbreaking |  1 |  2 |  3 |  4 |  5 |  6 |  7 | ... | 10 | ... | 20
Chance     | 20 | 27 | 30 | 32 | 33 | 34 | 35 | ... | 36 | ... | 38
Increase   | 20 |  7 |  3 |  2 |  1 |  1 |  1 | ...

(Values are rounded in the table, but not in game). The chance converges to 40% for high values of Unbreaking, with diminishing returns the higher the enchantment level gets, to the point where is negligible.

If you directly compare an Unbreaking III Diamond Chestplate to an Unbreaking IV one, on average, the former breaks after 529*100/(60+40/4)=756 hits, the latter after 529*100/(60+40/5)=778. That is only a relative increase of 3% more hits.

^{1 While the page has changed, the game mechanics have been the the same since at least March 2013.}