The representation of MOS size
I do not think there is any true standard about this. Hence it could be that a author does indeed represent the ratio of W and L with this.

However, I don't think that is generally the case (at least, I've never run into that). What it does mean is that the size in \$\mu m\$, in other words, that transistor would be 40 \$\mu m\$ wide and 4 \$\mu m\$ long. The reason it is kept as a ratio is because in the simplified equations for the \$g_m\$ of a MOS in saturation, the ratio between width and length is important, but at the same time, the Early effect is mainly determined by the length.

It is possible that an author is also referring to the size in \$nm\$. Context should make it clear which one it is.