Hi, altan
There is
τα έχει τετρακόσια, διατηρεί ακέραια τα λογικά του:
Mη βλέπεις που γέρασε· τα έχει τετρακόσια (from
here).
Of this phrase there can be various derivatives, all around the well-being of the mind. Here, it is possible that N.K. mixes two phrases:
Να έχεις το νου σου! (Be mindful) and
Να τα έχεις τετρακόσια! (Be well in mind) to create a mix meaning something more powerful. But it is also possible that he uses the number 400 only to show the extra mindfulness one must have...