Dictionaries, in general, will not incorporate tone sandhi rules into their pronunciations (of which Mandarin has quite a few)
Wikipedia says the following:
Mandarin features several sandhi tone rules.
When there are two 3rd tones in a row, the first one becomes 2nd tone,
and the second one becomes a half-3rd tone. E.g. 你好 (nǐ + hǎo = ní
The neutral tone is pronounced "low" when following the 1st, 2nd, and
4th tones, and pronounced "high" following the 3rd tone.[dubious ]
不 (bù) is 4th tone except when followed by another 4th tone, when it
becomes 2nd tone. E.g. 不對|不对 (bù + duì = bú duì)
一 (Yī) is 1st tone when it represents the ordinal "first," Examples:
第一个 (dìyīgè). It changes when it represents the cardinal number "1"
following a pattern of 2nd tone when followed by a 4th tone, and 4th
tone when followed by any other tone. Examples: 一个 (yī + gè = yí gè),
一次 (yī + cì = yí cì), 一半 (yī + bàn = yí bàn), 一般 (yī + bān = yì bān),
一毛 (yī + máo = yì máo), 一会儿 (yī + huǐr = yì huǐr).
The rules you need to learn yourself dictionaries will not help you out on this one.
So to answer your question anytime you see 不+4th-tone it will actually be pronounced 2nd tone+4th-tone