My players just survived an encounter of fighting off a small army of CR 4 creatures. We kept a running tally of how many of the creatures were killed before the players eventually drove the attackers off. Now I'm a tad perplexed. How do I calculate appropriately for the XP they gained? In the end they defeated 97 CR 4 creatures in a single encounter. I'm entirely lost on where to even begin to calculate their XP. Any help would be AMAZING! Thankies!
What level are they?
There's no hard rules for awarding XP outside of the standard encounter system. Theoretically, 97 CR 4 creatures is roughly equivalent to a CR 17 encounter -- but the system is not built to accomodate for this kind of situation, and I would not be rewarding them as a CR 17 encounter.
It depends a lot on how difficult the fight was for them and what level they are. If it was a tough fight I would probably be awarding them an APL+3 encounter assuming they are under level 14 (which should be a hefty sum of XP, but not so much it breaks the system). If they are 14+ or were in no real danger, this is not a difficult encounter for them and they probably should be just getting standard or even APL-1 XP.
First of all, what level are they and were there any NPC's helping? 1x CR 4 monster usually gives 1200 XP so it comes out to 116400 XP total, which is equivalent to between a CR 17 and CR 18 encounter. If you want to reward the players with XP for the monsters they didn't actually kill, just bump it up to the CR 18 or CR 19 figures of 153,600 or 204,800 XP, respectively. Using the CR 18 figure and assuming a party-size of 4 or 5, the CRB (page 398) recommends giving each player 38,400 XP. That's enough for a level 8 character to reach level 9, but less than a tenth of what would be needed for a level 15 to reach level 16 and only a drop in the bucket for a level 18 party.
Alternatively, if the fight wasn't actually difficult enough, in your opinion, you could have a reduced XP reward and keep the rewards story-based. Some GM's don't bother tracking XP at all, and just have the party level when they think a milestone has been reached. More details about your scenario would be nice. =]
EDIT: Aw, ninja'd while I checked the CRB tables. x.x
Either multiply the reward for a CR 4 encounter by 97, or rate the fight as an overall experience. Was it hard? APL+2. Epic? APL+3. Deadly? APL+4.