I'm answering myself, for future readers:
The 4DTv ratio cannot be converted into millions of years, as it is a relative measure of time and therefore can't be converted into an absolute one.
However, one can use the rate of substitution at neutrally evolving sites to determine age. Basically:
- parse a pairwise alignment in codons
- select only codons belonging to the fourfold-degenerate group
- extract third positions of each and count them (tot. positions)
- extract the number of positions which differ between the two alignments (substitutions)
- compute substitutions / tot. positions to get substitution rate
- compare it with a known substitution rate per position per generation time (in years)
The resulting number should be an approximation of how many years have gone by. Be careful, because this assumes a constant mutation rate and can only be used when under the assumption that no differential mutatation rates have been present among species tested (i.e. almost never).
If you can't assume a constant mutation rate per generation, then you can still get a very rough picture of the divergence time in millions of years, knowing that it is imprecise.