There is something to be said for the #4 hitter being able to drive in runners ahead of him with base hits, but maybe less than you might think.
I went to the lineup analysis tool at
http://www.baseballmusings.com/cgi-bin/LineupAnalysis.py. I plugged in a reasonable Giants lineup constructed around a hypothetical hole in LF, which I filled with either Adam Dunn (.280/.410/.556) or the OF with the highest batting average who might reasonably have been available to the Giants in the offseason: Rajai Davis (.319/.376/.435).
Here Davis has a 39-point advantage in BA, but a 34-point deficit in OBP and a 121-point deficit in slugging. The results are that the Dunn lineup is expected to score 4.349 runs/game, while the Davis lineup is expected to score 4.111 runs per game. Here Dunn is expected to be worth 0.238 runs/game, despite a lower batting average.
However, I thought that was a bit unfair since Dunn is clearly a better hitter than Davis. So I also took Pablo Sandoval (.326/.381/.545), who I had originally slotted into the third slot in the lineup, and swapped him to the 4 spot for both Davis and Dunn.
Sandoval has a much better batting average than Dunn (46-point advantage), and his OBP (-29) is similar to Davis'. Importantly, his SLG (-11) value is almost on par with Dunn. In fact, the Dunn/Sandoval lineup does score more runs than the Sandoval/Dunn lineup: 4.357, for a difference of +0.008.
But what about swapping the Sandoval/Davis lineup to be Davis/Sandoval? Here Sandoval has a small advantage in BA (7 points) and OBP (5 points) but a big advantage in SLG (110 points). The result is an expected 4.171 runs/game, a difference of +0.060.
So the Sandoval/Dunn swaps show that putting OBP in the 3rd slot and BA in the 4th slot gives you a small advantage. But the Sandoval/Davis swap shows that moving SLG from the 3rd slot to the 4th slot (everything else about the same) gives you a much bigger advantage.
And the Dunn/Davis comparison shows that getting OBP/SLG is
much more valuable than BA in the 4th slot. Ideally you would have them all,
but if you've got to make a tradeoff, OBP/SLG gives you the better return.