FrogReaver
The most respectful and polite poster ever
I think the "sneak attack" from the A5E wererats was the result of the advantage from pack tactics which allow them to add +2d6 damage.
This could result in 2 attacks at +(5+8) with advantage (plus expertise from flanking) dealing 3d6+2 damage each. That still might need a crit or above average damage to drop the paladin but he won't look good at any pace.
If the paladin engages one of the rats another just has to engage him so both would get pack tactics in one round (otherwise only one of them would get pack tactics unless they somehow coordinate and one readies his attack until the other has engaged)
What was the other wererat that granted the cleric's attacker pack tactics? Just a normal 1d6+2 hit and another one if the cleric just ran (and didn't disengage) don't seem to be enough to threaten him.
But nontheless... 4 CR2 wererats are a total CR of 8, which cost 4 encounter points (out of the 2 encounter points per day that 4 level 3 characters have).
And three other assessments from Sly Flourish's 5e Encounter calculator:
Encounter Assessment
Base Monster XP: 1800
Party: 4 characters, average level 3
Lazy Encounter Benchmark
Total Character Levels: 12 | Total Monster CR: 8
Potentially deadly if total monster CR ≥ 3
Potentially deadly single monster if CR ≥ 3
Assessment: POTENTIALLY DEADLY
D&D 2024 Method
Base Thresholds (level 3): Low 150 | Moderate 225 | High 400
Party Thresholds (×4 characters): Low 600 | Moderate 900 | High 1600
Base Monster XP: 1800 | Above HIGH
D&D 2014 Method
Thresholds: Easy 300 | Medium 600 | Hard 900 | Deadly 1600
Multiplier (4 customMonsters): ×2 | Adjusted XP: 3600 | DEADLY
So, even if the party was fully rested, and somewhat prepared things might have gone not so great...
Stumbling "blind" (the rogue "thought" that he might just kill one (or more) of those 4 guys so he charged into danger) and not really using tactics (at least I can't see much of it in the description) doesn't really help them.
2 of those rats might have been a better fight while also making pack tactics way harder for them to get... unless the room also contained "allied" normal rats that the wererats could interpret as allies even if those normal rats don't have that much personal impact on the combat.
Oh wow so A5e wererats are buffed. This was 4 creatures with advantage and able to deal 3d6+2 x2 damage at level 3, I think I could kill most 4 person parties with that.
Especially if they acted in a single initiative group.