Maximising the critical hit frequency
If you have around 15 luck, and you shoot some target without any critical frequency perks in your build your critical rate is 1/4 - meaning that you use 3 shots to load the critical bar, then the fourth shot is critical.
If you add in Critical Savvy level 3, your critical hit rate will drop to 1/3 - two shots to load, then crit.
If you have more than 20 luck, you'll reach the same 1/3 frequency with only level 2 Critical Savvy.
Four Leaf Clover has a chance of refilling your critical bar when you hit in VATS. It may, or may not trigger. When it does, it allows to load up Critical Bar even in one hit. Unlike Critical Savvy, which provides advantages you can count on, you can't count on Four Leaf Clover as much, but it can be vastly more powerful. The way the RNG works in calculating if shots a hit or a miss, its recalculated in an interval that might be longer than interval between your shots. Because of that, some of the effects may enter a 'unsynchronised' state, in which: Critical Hits might be prolonged if you don't cease shooting after sending out a crit, so that multiple projectiles get to the target as critical. What's more, some of these critical projectiles trigger the FLC, so that your crit-consuming shot is reloading your crit.
FLC additionally is triggered per-hit, meaning that multi-projectile weapons have increased chance of triggering the effect, making Critical Savvy redundant for shotgun-VATS or plasma-splitter-VATS builds.
So, to sum up the difference, Critical Savvy will consistently increase your critical hit frequency, while FLC will sometimes give you infinite crit.