Some form of Samba 4 would be in SME v10
Note there is big difference between standard file sharing et al as per Samba 3.x/4.x and the full Samba 4 + AD support.
Converting to S4 'as is' should not be that difficult. Running it as AD is... see the bug tracker for Gregs efforts in that direction. Full AD means a major rewrite of a large amount of code.
A few other points of note... we hadn't tested updating to Samba 4 on v9 as there were no packages available. RHEL finally built some for RHEL 6/SME v9 later last year I think. They have tested so check on the W10 issues... again see the bug tracker.
It would probably be possible to properly update to these on v9 (see the bug tracker) but it won't fix the W10 issue which is of M$s own making (and yes I'm sure they are making lots if $$$$ out of this little mistake!)
Yes it would be great to have v10 + Samba 4 + other goodness, but it needs time, bodies and lots of help.
Regrettably lots of people say we should do this that or the other, but when we ask for them to pitch in they never seem to have time or experience (we really need you time... we can help you with experience)
Personally I use SME for all sorts. Email, web, files, voip server, media etc. Real servers, VMs and cloud VMs, but have the benefit of having dumped Windows clients years ago. Best decision I ever made
So, if you want to continue using SME please come and help and make it the distro you want.