How does channel divinity work?
Clerics gain the ability to channel divine energy and depending on your Domain and level you can use your "channel divinity" for different abilities.
The amount of times you can use a "channel divinity" ability is limited, depending on your level, as stated in the PHB (pp. 58f).
Can a 4th level cleric give a character 20 HP?
Yes, a cleric with the Life Domain can restore 5× his cleric level hit points using channel divinity for his Preserve Life ability.
This is limited in several ways:
The cleric may choose any number of creatures within 30 feet range and divide these hit points among them.
It cannot restore hit points above the half of the target creatures hit point maximum. (The relevant wording is:
This feature can restore a creature to no more than half of its hit point maximum.
This means you can restore any amount of hit points (up to 5x cleric level) as long as the current amount of hp of the target do not exceed half of the hit point maximum of the target.
It does not work on undead or constructs
Regarding these limits:
Target creatures hit points: 20/50
The half of the maximum would be 25, so preserve life could only restore 5 hit points for this creature.
Target creatures hit points: 2/24
The half of the maximum would be 12, so preserve life could restore 10 hit points.
Combining both examples, when both creatures are within 30 feet of the cleric he could restore 5 hit points to the first and 10 hit points to the second creature.