It's quite common to see suggested guidelines of at least 20 or 30 buyins. Playing 4NL, that means your bankroll should be at least $80 or $120 respectively.
An aspect that doesn't seem to be addressed as often is why one should pick 20 instead of 30. Or some other number. Basically, if you play a relatively low-variance style of game, you can use a smaller guideline figure. If you play in a way where you swing up and down more, you should use a higher number in order to buffer yourself against the larger downswings that you're likely to experience.
Also, play at levels that are in your comfort zone both relative to your BR and emotionally. For example, if you feel uncomfortable bringing more than $20 to a table, you shouldn't play above NL20 no matter how large your BR is. The exception to this is playing higher in order to get used to it so that eventually, when you've desensitized yourself enough so your discomfort level doesn't affect your game, you can move up.