They don't. They don't take 4 years to make an album. That is a very misleading statement. They ONLY way one could think it takes U2 4 years to make an album is if one expects U2 to start making a new album the second they're done with the last one. You do not measure from the release of album A to the release of album B to measure how long it takes U2 to make an album. That is absurd. You measure from when they started making the new album to when the finished making the new album. Logical enough, yes? Now, we have no real way of knowing exactly when they started making the album that is going to be released in a couple weeks, but let's just say for the sake of a hypothetical that they start making the new album as soon as the tour for the last album is over. Let's start from the Zooropa-Pop transition.
ZooTV finishes in December 1993, Pop is released in March 1997 = 3 years, 3 months
Popmart finishes in March 1998, ATYCLB is released in October 2000 = 2 years, 7 months
Elevation Tour finishes December 2001, HTDAAB is released November 2004 = 2 years, 11 months
Vertigo Tour finishes in December 2006, NLOTH will be released March 2009 = 2 years, 3 months
U2 took less time to make this record than they did making the last three. And, in fact, ATYCLB and HTDAAB took less time to make than Pop did.
And this is assuming that they started making each album as soon as the previous tour ended, which is probably not the case, and this is not taking into consideration that each record, with the exception of Pop, was completed 1-2 months before release(even more than that for Bomb, if I recall), so those numbers above are realistically even smaller.
This complaint that people have about U2 taking SO long to make a record is based in false and inaccurate measurements.
The reason there's so much time between each batch of new U2 music is because they tour more than a lot of other bands do.