Yes, 4-2-4 is correct. According to the sandhi rule of 一
i.e. 一 should be pronounced in the 2nd tone before the 4th tone.
And the sandhi rule of 不
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.