There's a long side and short side on the common pallet, so depends how it's oriented how to lift. short side - 42 is good, 36 won't cut it. Long side - 48 reqd.
If it's not too heavy then the boards won't separate with the 42 but the 48 covers it.
Not exactly correct...
I've been operating forklifts (standard riders, narrow aisle, and turret trucks) in a warehouse environment for the last 45 years. A standard 4-way pallet is 40" wide by 48" long with the stringers being 48". Typically there's 3 stringers, sometimes more. They have cutouts for the forks, thus the 4-way designation.
You want a minimum of 75% fork penetration so technically a 36" fork can be used on a 48" pallet but loading and load center become more of a factor.
From the short side, you can use 36" forks through the stringers (only 2 however) but again loading becomes an issue; 42" forks will grab all 3 stringers. Most forklifts being used with standard pallets are equipped with 42" forks since they work from either side without excess blade length hanging out the blind end.
The main reason we use 42" forks in our warehouse is not the pallets, but the pallet racking we utilize. A forklift with 48" forks cannot safely swing in our 9' aisles.
Now, forks on a tractor are very different (in how they're used) from forks on a lift truck, but the principles are the same. In my opinion, when working with standard 4-way pallets, 42" forks are the preferred length. 48's will work but limit you from the side. 36's are too short to handle everything.