Elaines very helpful guide stated that for v4 LM recommends using Win10.. 

May this indicate either that DX12 is supported or is planned for a point release, or have I missed previous news about this? (very likely!)

If Win10 is only "recommended" by LM that defintely means it´s compatible with prior Windows versions 7 and 8.
So unfortunately V4 is obviously based on DX11 like V3.

DX12 will maybe come with v5.  Converting a 3d engine to a new API does not automatically mean there will be more performance available, so don't pin your hopes on it.  There's a lot to be excited about with v4.  I held off on getting v3 because I said to myself I needed 64-bit before I would buy another sim.  

Interesting... Does DX12 bring any specific improvements which could relate to simulation games? (generally curious).

4 minutes ago, Anders Gron said:

Interesting... Does DX12 bring any specific improvements which could relate to simulation games? (generally curious).

The following is true for any API:

Your use-case and your code need to be finely tuned in order to take advantage of new features which could, in turn, alleviate bottlenecks i.e. increase resource utilization i.e. potentially increase performance.  If a game (sim in this case) has inherrent serial processes which cannot be parallelized or sent to a co-processor/ASIC (such as a GPU) then your code is still going to be held back by the CPU.  Sadly, flight simulators are notoriously serial in their nature.  I believe this could change over time, especially as it pertains to scenery engines, lighting, environment (weather), and A.I. but there is still a very large and central block of code that can only run in a single thread and that will probably never change.  

Thanks for the explanation.

From what I have gathered so far, it appears that the new bottleneck in V4 is VRAM. The thing with DX12 is that the VRAM on multiple card in SLI are combined i.e. if you have 2 cards with 10GB each, you get a total of 20GBs available, With the same setup under DX11 you only get 10GB's

 

EDIT: well I won't say who told me but I'll say that a very reliable source confirmed it is not DX12

18 minutes ago, Avidean said:

From what I have gathered so far, it appears that the new bottleneck in V4 is VRAM. The thing with DX12 is that the VRAM on multiple card in SLI are combined i.e. if you have 2 cards with 10GB each, you get a total of 20GBs available, With the same setup under DX11 you only get 10GB's

 

EDIT: well I won't say who told me but I'll say that a very reliable source confirmed it is not DX12

 

VRAM *can* be a bottleneck in v4, if you adjust the setting which controls scenery texture sharpness rendering distance (previously called LOD RADIUS, not sure if the same term is used in v4)

58 minutes ago, Avidean said:

From what I have gathered so far, it appears that the new bottleneck in V4 is VRAM. The thing with DX12 is that the VRAM on multiple card in SLI are combined i.e. if you have 2 cards with 10GB each, you get a total of 20GBs available, With the same setup under DX11 you only get 10GB's

 

EDIT: well I won't say who told me but I'll say that a very reliable source confirmed it is not DX12

Dave I agree, its a positive step that LM recommend using Win10, probably to build the user base for a future DX12 update. 

1 hour ago, Avidean said:

EDIT: well I won't say who told me but I'll say that a very reliable source confirmed it is not DX12

Your "very reliable source" is absolutely correct...

I got the impression from what I heard that there is a considerable development overhead involved in switching to DX12 and the ability to use the VRAM on all cards would be the developers responsibility not the Hardware drivers.I am guessing that switching to DX12 was possible and will be possible in the future but I am also guessing that is will also spell the end of Addon up dates for free since probally all addons period will require major work to remain compatible to a DX12 P3D. Perhaps that will be the big move for Prepar3d V5