It depends on your park, your opponents' (particularly division rivals') parks, and your opponents' (particularly division rivals') starting pitchers.
Firstly, as with the rest of your team, you want to tailor your team to your park. If you chose a lefty park like (particularly) Progressive, Camden, Citizen Banks, or Dodger Stadium, you will need
more than 4 lefties in your lineups...at least against righties. If you choose a neutral park it's
not inherently important to your tailoring. If you choose a righty park like Target or Rogers, you may want even
less than three lefties in your lineups.
Secondly, scan the league. If one or more teams in your division have one of the above mentioned lefty parks, and/or there are many such parks in the league,
you will need to be able to compete in their parks. You can do this by either having
4 or more lefties in your lineup or having lh bench players who can replace one or more of your righties in these parks. Some players good for these situations are Chisenhall, Boesch, Garrett Jones, Parmelee, Gillaspie, and Florimon.
Finally, scout your division rivals' and (to a lesser degree) your league rivals' starting pitchers. If your division rivals load their teams with sp's strong against Rhs and weak against LHs, you will also want to be prepared to play 4 or more lh hitters. Even a Target team in this situation should "arm themselves" to be able to do so. As in the scenarios above, you will be balancing prepping for your own park against prepping for others. These aren't magic formulas, just principles that have usually worked for me...not so much this year...