I just got my 404sx and will primarily using it to record live vocals/keyboard shit through a bunch of pedals. How can I listen to a previously recorded loop while recording a new one? or is there a metronome that i can use built in? i couldnt find anything like this in the manual. how am i supposed to record live loops that will actually be on time if i cant hear my previous loops while im recording them?

sorry man, you can't do that with a 404. there is no way to get a sound out of the machine when you record a sample.
The 555 seems to be better for you... It has loop capure, so you can listen to previous recorded and stored loops while recording a new one.

i have my 404 going into the 303 so I can listen to a metronome or recorded loop while recording stuff into my 404. works also for resampling.

Maybe you can work around that by recording your previously recorded material to a tape or something and let it play from there while you're recording the next take.

what i do is open any software that can host vst's, set the tempo at whatever it is on the sp, and record from there then either export as a wav or just record straight into the 404.

You can set a count-in to get a feeling for the tempo, but I don't think you can set a metronome when sampling. I also think, depending on how many measures you're sequences are going to be, that the SP-404 was designed mainly around sampling short loops or live play.

To set a count-in (with metronome sound actually) for sampling; press [start/end/level] and [rec]. Then it'll go 1--2--3--4--rec and record at 'rec'.
To successfully use this, you'll have to set a correct BPM for what you're going to sample or the count-in will be too fast/slow. But you'll figure this out.