i am not really sure how you use a 4-track cassette recorder. do you just put the SP on one track and record from the SP to cassette? are these other 3 tracks useless if i only use one sp303, no instruments, no microphone/rapping.

thanks in advance
cheers

Some people isolate elements of their track (drums, bass, main rhythm, vocals etc etc) and each element is on its own track so it can be individually EQd later. This requires some work arounds using the SP fx to mute parts of your beat. Some people just use 2 track (left and right stereo) into their cassette recorders just for the analog warmth in their sound..... How's about you figure out how you want to use it and then ask a question about that Lastly, a 4 track is 4 mono tracks or you can make it 2 stereo tracks. 2 stereo tracks would require hard panning.

if i want my drums on track 1, my bassline on track 2 and my sample on track 3, i need 2 more devices, right? for example my laptop and a walkman

Generally a 4 track cassette recorder will let you overdub. So you could do it only using an SP and the recorder. Use an effect like isolator to mute the pads you don't want to trigger in your sequence. Rewind the tape and overdub the next part using the same technique. It's like building a beat the original "resample" way.

thank you that is the information i needed

You will def run into sync issues though if you try to use the sequencer to rec into the cassette recorder. If you play all the parts live it is easy you can just redo if you mess up. But unless you use a click at beginning and i think even then it is hard to get all tracks to stay in sync. Just a heads up.
Have fun!
C

bounce sand mixdown, mixdown and bounce
ps
in the 90s i used to leave a track for a "invented metronome" (generally it was a lick drum looped at the bpm needed) and then tracked live drums, guitar, samples etc i hadn't got a sequencer.
then my recorder accepted external fx (i stil have it, it's a yamaha mt120) and i made bounce of tracks to make room, and doing that i put fx on that bounced tracks...generally i left last tracks for the voice