Nope, ST4 does not have an Import feature.
But, ST4 can load / use ST3 instruments (both ST4 and ST3 "sound engines" are built into the ST4 software). If you already have them working in ST3, then you should be able to add the ST3 library path to the Disk Paths in ST4's Preference Settings, rebuild the cache database and use them in ST4.
I am not sure what you meant by "and it won't sync with ST3". Can you clarify?
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For other readers, there are a lot of SampleTank expansions on sale right now at
https://esoundz.com/landingpages/sampletank.php You'll need to install/authorise SampleTank Custom Shop (i.e. ST3) and Import them into the ST3 library, as many (all?) are in the ST2 instrument format. Oldies but goodies. Then add the ST3 library path in ST4's Settings, as above.