My suggestion -- reconsider your approach. 4096x4096 is the maximum texture size allowed by Unity.
A single 4096x4096 Truecolor texture is 67 megabytes of video memory. 88 megabytes if you've used mip-mapping.
Your 83 textures will take up 139 megabytes of memory at minimum, 183 megabytes with mip-mapping.
Needless to say, that's excessively large.