On the Retro-Tink 2X - I would email him directly to ask and be sure. Going through PayPal should give you a bit of support in case anything goes wrong.
As far as I know, the OSSC is only available at VideoGamePerfection.
The OSSC only handles RGB and component inputs, so be aware of that.
Do check to see if your 4k TV will handle the resolutions/timings that the OSSC will throw at it - see this forum thread.
In general, most LGs and TCLs seem to handle it just fine, but OLEDs have some difficulty.
However, you won't be able to use the N64 easily, unless it's RGB modded.
In that case, the RetroTink will cover about everything else that the OSSC won't.
Not sure how well the RetroTink 2x is compatible with TVs. Hopefully someone else can speak more about that.
Thanks. Seems like the move will be to go with the Retrotink2x at first, and eventually add the OSSC into my "stack," if you will. Looks like VideoGamePerfection isn't shipping outside of the EU quite yet anyway, and I'm in the States.