I have come across this NACA profile
NACA 0005.4 mod
and it is not clear how to interpret this airfoil. Is this considered a 4 digit NACA profile? What does the number after dot mean?
Yes, it is a valid airfoil identifier. First two zeros indicate it is a symmetrical airfoil with no camber. The next two digits mean it has a five percent thickness to chord length ratio. The number after the decimal indicates a slight adjustment to the maximum thickness and location thereof. Instead of being 5% thick, this airfoil is 5.4% thick. There is quite a bit of very good info on NACA airfoils here. This airfoil is mostly used on helicopter blades, far as I can see.