You don't get a 5th costume slot at level 50.
In order to get the 5th slot, you must go see Annah in Croatoa (near the station) and give her 4 bits of Halloween salvage (obtainable from the event or the Auction House)
In Villains, the costume unlock person is right next to the SG base exit in Nerva
"Idealism is such a wonderful thing. All you really need is someone rational to put it to proper use." - Kerr Avon
Myopic Aardvark on Twitter
Wow - a 9 minute response time. Much impressed and thank you! 
And in case you are wondering... the salvage needed is:
Stateman Mask
Lord Recluse Helmet
BaB Gloves
Hamidon Halloween Costume
Golden-Phoenix - Lvl 50 Fire/Fire Tank
Oodja Nikabolokov - Lvl 50 SS/WP Brute
Baby-Phoenix - Lvl 50 Peacebringer
How much wood would a woodchuck chuck if a woodchuck could Chuck Norris?
And since halloween is less than a month away, don't go to AH and but it now, but wait instead! 
Thanks guys
Is there a definitive list of new Halloween badges about yet?
Golden-Phoenix - Lvl 50 Fire/Fire Tank
Oodja Nikabolokov - Lvl 50 SS/WP Brute
Baby-Phoenix - Lvl 50 Peacebringer
How much wood would a woodchuck chuck if a woodchuck could Chuck Norris?
[ QUOTE ]
Wow - a 9 minute response time. Much impressed and thank you!
[/ QUOTE ]And to add to the other posts... you cannot use standard keys to change into your 5th costume. You need to either type /cc 4 or set up a bind or macro.
Costume slot 1 = /cc 0
Costume slot 2 = /cc 1
Costume slot 3 = /cc 2
Costume slot 4 = /cc 3
Costume slot 5 = /cc 4
Main 50 & Badger Nine Claws Claws/Regen/Body
Other 50's Ph10x Brute Unjust Law Tanker Samantha Urai Scrapper Codename 1250 Scrapper
Cepheus Defender Nilus Scrapper Electro Field Blaster S.PONGE Tanker
Hi all. Following my first character (Yay Miss Freeze!!) getting to level 50 - can anyone tell me how to unlock the final costume slot?