EDIT: (I just discovered SGSSAA isn't worth running with DirectX11)
DX 11 MSAA is way worse than DX 10.1 MSAA in GTA 5 for some reason. If you are OK with losing tessellation and grass, you can then use Nvidia inspector to enable SGSSAA (you might be able to do it with AMD cards as well) which looks great.
I think 4x MSAA + 4x SGSSAA on DirectX 10.1 looks better and is less performance hungry than 4K with no AA.
FXAA can be used to perfect it, but at this point it's better not to use SGSSAA as well as the ENB fix. FXAA is great but does look a little blurry in the far distance.
Of course 8x SGSSAA would be the best but 8x MSAA is already a killer for most of us.
Hope this helps you.
Here are some 1080p screens, please note Anisotropic Filtering has been set to 'High Quality' 16x via Nvidia Inspector (clearer textures).
Here is a screenshot comparison link for DirectX 10.1 and DirectX 11 both MSAA 4x.
Something seems to have broken the MSAA in directx 11, there are far more jaggies everywhere.
Here is DirectX 10.1 MSAA compared with DirectX 11 MSAA with the ENB fix.
Now with Boris' Fix (ENB), the MSAA is far better. It's almost the same as DirectX 10.1 MSAA as you can see... apart for some distant jaggies such as the palm trees' foliage and the top of the billboard.
That's simply R*'s fault and this is nothing Boris can fix.
We can still improve the MSAA with SGSSAA.
To enjoy SGSSAA you will need to run DirectX 10.1 MSAA.
Here is comparing DirectX 10.1 4x MSAA (already great) with DirectX 10.1 4x MSAA + 4x SGSSAA (even better).
With SGSSAA, there are less jaggies on the road paint and the hanging electronic cable, no white pixels on the car and palm tree trunks, and reflections, fences and balconies have more detail.
It is possible to run SGSSAA with DirectX 11's MSAA but you will need to run the ENB's fix and even running, the difference is minimal compared to DirectX 10.1's SGSSAA.
Here is a screen of my Nvidia Inspector, the multiplier of SGSSAA is set in concordance with the MSAA multiplier in game.
Verdict: Best way to get rid of jaggies is running SGSAA on DirectX 10.1. If you don't want to lose DirectX 11's features, the MSAA fixes need to be enabled on Boris' ENB but
there will still be more jaggies than with DirectX 10.1's MSAA with and without SGSSAA.