Technically, the parameter of interest should be the number of read/write heads rather than the number of platters.
Typically there would be one R/W head per platter surface. A platter has two surfaces, so if both surfaces are used for data storage, then the number of R/W heads is double the number of platters (for the typical case).
I haven't opened up a HDD in a long time, but in the past I have seen HDDs built with unused surfaces and/or platters. Presumably these were platters that had only one "good" side and installed in a low-capacity model that was otherwise identical to a high-capacity model.
Increasing the number of R/W heads in a HDD is considered a performance benefit, since more data would be in each cylinder and thus can reduce seeks.
The typical tradeoffs for more R/W heads is added cost and reduced reliability (added electronics, more mass to the head actuator) and increased susceptibility to a head crash.
if you have a 4TB hard drive with one solid 4TB disk and you have another 4TB hard drive with 4 discs, does the one with 4 discs have a greater chance of failure?
I would consider a 2-head (single platter) drive to have less chance of failure than a 8-head (4 platter) drive. But (assuming similar areal densities) the 8-head drive should have better random access times.
How about 2 platters vs 3 platters?
Until someone quantifies the increased risk of failure due to the number of R/W heads, you shouldn't base reliability and/or a purchase decision solely on the number of platters and/or R/W heads.