What are the tones for 不 and 一 in the expression 不一定?
Yes, 4-2-4 is correct. According to the sandhi rule of 一


  在四声前边要读二声


i.e. 一 should be pronounced in the 2nd tone before the 4th tone.

And the sandhi rule of 不


  “不”在一二三声前面不变调，读四声


i.e. 不 doesn't change the tone (the 4th tone) before the 1st/2nd/3rd tone.

BTW: 2-2-4 sounds weird to me.

Reference: 一字与不字的变调问题