damn I expect by that time difficulty would be through the roof. If it does release a d4 even though I can't find a d1 or d2 would be nice to see what the payback period looks like.
There won't be a significant technology update until 7nm asics are available, some time in 2019. TSMC's 7nm is still in early risky production and at it's not available for the asic market.
There might be a revised version of D3 (say D3+) somtime next year but it won't make a huge difference.
So, my guess is the D3 will be like the L3+ or the S9