Wait, why are the masks RGBA instead of grayscale?
1st one is typically a brush-in one - basically just Alpha brushed in from various alpha-brushes over the map
2nd can be a noise function, where the 1st alpha mask is multiplied by the second to get a brushed noise, and the noise might be useful for randomly-shaped patches of different-colored grass in a scene
3rd might be a shadow mask, where any objects "stamped" on top apply a default shadow at a given angle, and that default shadow is written into this channel. Then the map-maker can alter those if (for example) a shadow hits a wall.
Then there's the shader mask - so you can select a given shader for a given area of the map. These'll all be batched up later, but if you want flowing water, brush in where you want the water shader to be applied. Same for smoke, lava, etc. etc.
A 4kx4k map layer section, in RGBA is 64MB. Each layer on the map has multiple types of mask applied (brushed alpha, brushed noise-function,...) so there's another 64MB of XXXX data per layer. A map can have (say) 10 layers on it, so we're up to 1GB, and then there's the data for ephemeral layers ("stamped" images at co-ords and scale, alpha,rotation). The stamps themselves are usually stored in an 8kx8k layer so that's another 256MB
When your 'save' action is 1.5GB of data, a 10Gbe network is much preferred over a 1 Gbe network...
All of this is rendered down for production of course, but during development, each layer is kept separate along with its masks.