I appreciate your circumstances- one fight/day, one session per month, but you're letting the players off the hook with the encounter level +4.
I am a sad DM that tracks lots of things, so I can tell you now that I have played out 189 combats with my present party to date, I can tell you about each and every one of them, I know... I know...
So I've tried a few things, such as throwing six very short encounters at the group without letting them have a short rest... that scared the bejesus out of them. That was in Thunderspire Labyrinth...
Couple of Gnolls and a few Hyenas, easy work. Smash and the next lot is on them- more Hyena's and a Dragon- just get through that and here come more Gnolls, a hoard of them- the PCs have to flee at top speed, then part of the walkway they're on unpeals and begins to tumble into the chasm- which brings them out in range of a bunch more Gnolls with bows, so now the PCs are being shot at, cannot retaliate and are having terrain issues (ongoing Skill Challenge), and on it goes... Even to the point they reach a 'safe' zone (they think), only the ceiling collapses suddenly- the dragons back and the Gnolls drop back in and they're on the run again.
After five encounters, with no short rest, they run out into the open and safety.
That's when the Demonic Scourge and a pair of his Demon friends hit them.
My point, each encounter was level, or -1, -2, -3 even but six in a row and in one encounter, maybe 2 hours play we have Indiana Jones and his friends versus the world, oh and then the path they've been following is destroyed- no way back. Oh and if they defeat the monsters too quickly and they think they're going to rest- don't let them, have eight more monsters ready to go... rinse and repeat until they're screaming over Skype at you to make it stop, or running in fear with their 18 hit points left.
I've done it a few times to be honest, PC Rogue wants to get pay back on NPC Rogue- the other PCs are convinced (somehow).
PCs beat guards to NPC Rogue lair- except the one that runs inside- PCs head straight after (easy encounter afterall), straight into Encounter Level +2 or +3 with lots of Minions that get in the way and artillery around the outside, that slows them up until the invisible/concealed assassins arrive.
Then when they get done, another door opens and we're back into another encounter, with lots of fiddly little stuff, the PCs think they're gods gift and brazen it out.
Then the boss arrives.
PCs left with no healing, all battered and broken- no-one down admittedly but they were swearing and cursing.
The boss cuts a deal, and the PCs go home, tail between their legs.
I have a story I'm working on with my players (they don't know the story- I do), if I want to scare the bejesus out of them... then I do.
The players have just reached Paragon Level, I've thrown two level 17 encounters at them already (actually they were at level 9 & 10 respectively when I threw them at them). Winning sometimes means surviving, not getting the girl or saving the world.
Last one, encounters in a day, as in between extended rests- 13, that's our longest day; 13 encounters in a day, don't get me wrong the PCs are pretty hard core- not power gamers but willing in-game to try things (not just fancy encounter powers with dumb names) like leaping of houses into combat, attempting to drown creatures, riding crocodiles... you know the kind of thing- I have an anything goes action hero unwritten policy.
Not killed one yet. Two failed death saves, lots of times... never the third, 61 sessions in game, nearly two years.
I love it.
They love it.
PDR
If no one's scared, dead or whatever it is you want to achieve then the encounter doesn't stop there, a door opens and in walks... and then... and then... make them run away a couple of times, that makes me smile.