The OSV is an excellent SIP server because it has to be. If you want SIP subscribers or SIP trunks then OSV has got you covered, cannot deny it's a fantastic machine for that. But if you want something else like attendant consoles, analogue extensions, PRI TDM trunks, you need to start adding on the external boxes. Does anyone really want floor to ceiling Mediatrix units with all that extra protocol conversion, extra management GUI's? It's not ideal. So you can't really compare 4K and OSV as like for like. Some scenarios better suit OSV, some (like anything where there is more than a little TDM), better suit 4K. And if you want SIP on 4K, it's no problem. Native SIP trunk support to third parties on 4K is excellent, as supported RFCs confirm. Where there is a difference in SIP support between OSV and 4K is SIP subscriber. Cornet protocol for phones as provided by 4K is hugely feature rich, and if you have a 4K with Openstage TDM's or HFA's, there's really no reason to use SIP subscriber, you won't gain anything. Internal politics restricted the development of SIP subscriber features on 4K when SIP became popular because there was concern that it would interfere with OSV sales. So 4K could provide SIP connectivity for subscribers, it worked, but features were limited. No real issue as it didn't need them, use HFA instead. But it would have been nice to have those boxes ticked. This development restriction seems to be no longer the case though and SIP subscriber features have taken a leap forward with the new UFIP configuration method for providing SIP subscribers on 4K from V7 R2, I guess they saw the light. There are still feature restrictions of course but it's not like it was, and it'll only get better.
For a customer invested in 4K, EOL statement would not concern me. Development for both OSV and 4K has been rapid, and with both of them now supported in ESX, all sorts of interesting things could be done. I'm sure a suitable upgrade path for whatever system is then current, regardless of what it's called, will be provided.