Sayang, I totally understand your worry, spotting coklat yang berlainan dari biasa memang boleh buat kita rasa confused!
Okay so dari apa yang you describe, few things stand out: kalau biasanya your spotting langsung tukar jadi fresh blood flow, tapi kali ni jadi cair-cair je selama 4 hari, tu memang pelik sikit. Spotting coklat boleh jadi dari sisa darah lama, hormon changes, atau even implantation bleeding kalau you're trying to conceive, tapi memang difficult to tell without proper checkup.
The tricky part is UPT negative but period also berubah behaviour... someone in my mum group ada experience similar, spotting coklat sikit-sikit, test negative, tapi hormones dia memang berubah. Kalau period you biasanya very predictable, maybe tunggu 2-3 hari lagi then retest, sebab sometimes the hormone levels still too low for UPT to catch.
My honest take: jangan tahan-tahan lama, better go see your gynae or clinic soon. Bukan emergency tapi worth checking sebab pattern you ada tu bukan standard, doctor can rule out anything else and confirm either way. Jangan stress je while you tunggu appointment okay!