What is a unique CR4 monster?

[RPGaddict28]

Some backstory first. My campaign takes place in a world where there hasn't been monsters for hundreds of thousands of years, or in game terms, no non humanoids, (magical or not) animals, vermin, good alligned fey, and probably some innocent enough creature type that I forgot are naturally there. All other monster types are either in their native plane, or in an inhospitable landscape, surrounded by mountains, and locked off with a magic spell. When the campaign takes place, rumor has had it that monsters had escaped, and are in mainland.

Now, want my players to actually encounter a monster. I want it something memorable. Something that they'll never forget. But, I have a problem, I don't have time to go through the 6 or so Monster Manuals, and, I don't want to accidentally send them an over or under CR'd monster. So, I'm gonna need your help to find a good monster for me.

[EvilAvocado]

If you give us some information about the party (their makeup, levels, equipment, optimization level, etc) It would help us pick a memorable encounter.

[RPGaddict28]

If you give us some information about the party (their makeup, levels, equipment, optimization level, etc) It would help us pick a memorable encounter.

A ranger, paladin, rogue, wizard, and a DFI bard at 3rd level, nothing special gear wise, except a +1 greatsword, and mild optimization to above mild but not spicy.

[ZamielVanWeber]

FF: A Haraknin (under canomorph) with 1 class level and CR4. And let's face it, having your opponent go "forget this" half way through a battle and turn into a hell hound has some potential cool.

[Biffoniacus_Furiou]

Forest Troll, MM3. It's durable enough to last a few rounds despite ~3d6 fire damage per attack per character from DFI, and it's a fairly dangerous opponent. Keep in mind it's a natural hunter, and would probably try to ambush them.

[Vanitas]

I'd use a higher CR monster. Your party is going to destroy a CR 4 monster rather easily.

[RPGaddict28]

I'd use a higher CR monster. Your party is going to destroy a CR 4 monster rather easily.

Well, what CR would you recommend? 5? 6? Or higher?

[Thanatosia]

Another big point - assuming you have a party of multiple players (edit: and it looks like you have a 5man group), you almost never want to try challenging them with a single monster. Action Economy makes single-boss vs group balancing almost impossible to do right in D&D, most attempts to do so tend to end up with a monster that one shots a PC before going down to the combined focus-fire of the rest of them over the next turn. It almost always works out better to use groups of monsters... you can have a boss, but make sure he has mooks rather then trying to balance him as an encounter vs a full party by itself. It's not 100% impossible to do a single monster vs full party encounter, but its really really hard to balance right and you sound uncertain of your ability to fine tune to that degree.

[Sir Chuckles]

Well, what CR would you recommend? 5? 6? Or higher?

A party CR vs. That +1-3, depending on optimization, is a good bet for a memorable encounter.
A single CR 4 will get brutally murdered, and will only award 250 exp.

1 CR 5 and 3 CR 2s will last much longer, and award 720 exp.

Think Goblins, Orcs, or Half-Orcs with class levels, and a big dude that they listen and answer to, such as the Troll. And furthering the monster super-Alcatraz escapees theme, you could have them as stealth based classes, with some arcane flavorings.
A Goblin Spellthief teaming up with a Troll Ranger sounds fun to me.

[Vanitas]

I would avoid creatures with class levels, since they will feel like they are fighting wizards in rubber suits. Go with monsters, specially monsters with unique abilities.

How about a displacer beast and a bunch of rust monsters? How about a ravid and a bunch of animated objects?

[Maginomicon]

You might want to look through the Dungeons & Drawings image blog (http://dungeonsanddrawings.blogspot.com/) for ideas. They frequently forbid the artist from seeing the original art for a monster, only giving him the stat block and ability descriptions. The artist then takes that into a refreshingly different artistic direction than the original.

You can do the same thing with encounter design. Take a monster's stats and abilities and then reimagine what it might look and act like.