I think 5th base refers to anal sex, which really is just another version of 4th base.
First base is kissing, feeling up, petting, but no direct and prolonged genital contact.
Second base is hand and fingering, and can sometimes include (if you're into that sort of thing) anal fingering. It should be noted at this point that I am not into anal fingering!
Third base is oral sex. Licking, sucking, and like second base, it has an anal equivalent, often referred to as a rim job.
Fourth is just sex. It can include anal sex. Basically, if the penis penetrates any orifice below the waist, sex is occuring.
The hard thing with this is when you consider homosexual relationships. It's all good for two gay men, but when it comes to lesbians there really is no fourth base to speak of. I think that toys should be able to count as a penis, and thus fulfill any of the bases in the same manner.
That's my personal view on the issue. Hope it was coherent