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An additional four 150 feet (46 m) diameter pads are planned to support the simultaneous recovery of additional boosters of the Falcon Heavy.
on Cape Canaveral SpaceX landing Complex/Zone 1 wiki.
The falcon heavy having 2 booster, one would think having 2 contingency landing pads would be enough. (See the pictures below for illustration).
SpaceX demo heavy video, img source
I found the answer from a SpaceFlightNow article.
The contingency pads would only be utilized in order to enable the safe landing of a single vehicle should last-second navigation and landing diversion be required,
Bottom line, there are 4 because they are only to be used if for some reason the booster is off target, and one of the contingency pads is closer. You need 4 to spread them far enough apart to make it likely to always have one near.
Originally it was thought that the Falcon Heavy landing would use some mechanism to separate in time the landing of the two core boosters. Perhaps, slow one down, allow one to reenter a bit faster to buy enough time for one to land, perhaps let the dust settle down, before the second one landed.
Elon tweeted that he expects them to land simultaneously.
The contingency pads in the EIS apparently have yet to be built, as late as the CRS-9 landing at LZ-1. So it is unclear yet what the purpose would be, although as a divert location it does make sense, as is stated in Pearson's answer.
An article in the Orlando Sentinel indicates they are looking for permission to build two more pads. It is unclear from the article if they mean the contingency pads, or more pads. The article references within the LZ-1 complex, but never calls them by the contingency pad names.
