Hey Retrax,
the result you’re looking for can be achieved by repeating what you already did with the Brick texture, changing the its Scale to be one fifth of the first one and then combining the two. Something like this:
Since the grid only uses one color for the lines and one for the background you can use the ‘fac’ output (Mortar = white, Brick = black) and combine them using a math node. We can assign the colors with a MixRGB node and using the the combined mask as input for the factor. While I’m using a newer version of Blender, all of that should be possible in 2.79. Hope this is helpful.
Because your example isn’t just a continuous grid, but has a border I also made a setup that includes a border in case that’s something that might interest you:
