Why are both tier 4 now?
To make way for the true Tier 5 system - finst.
An equivalent would be Ireland where routes 1-50 are National Primary, and 51-99 are National Secondary (though the distinction, unlike Finland, isn't signed) and 100-999 are Regional.
Valtatiet (1-39, white on red) are National Roads / Staatsstraßen
Kantatiet (40-99, black on yellow) are Main Roads / Hauptstraße
Seututiet (100-999, black on white) are Regional Roads / Regionalstraßen
While they are two different systems, they are both of tier 4 level.
And what's different for Åland?
The second tier roads are much less important than on the mainland.
Their system names are different, which I'll fix (might go with Swedish names, rather than the Finnish names I give - more research needed as of now).
1-digit roads (Päätie) have their mainland equivalent as Valtatiet (though are probably the equivalent of Kantatiet in importance) - white on red
2-digit roads (Tie) have their mainland equivalent as Yhdystie (though are probably the equivalent of Seututiet in importance) - white on blue