I don't have a timescale for the cutoff of PT3 support yet.
Have you opened a Support Ticket about your conversion problems?
If not, please do - if you have hands which don't import correctly then we'd like to get that sorted out. Probably the best way to proceed would be to do a complete export from PT3 and try importing each site folder to PT4 individually - when you get some files which don't import correctly attach a few examples to your support ticket so that we can investigate and apply a fix as necessary.