Best estimates on 3.x LTS for new project work

Sorry if this has been kicked about elsewhere...

I'm a bit of a noob when it comes to full stack web dev so I'm learning as I make mistakes on my local server .  I'm specing a rather ambitious CI project that will occupy me for at least a year (too many grandkids soccer games) and I am seeking some advice from the seasoned hands here. 

Initially my project code will be speculative as I refine the application - I anticipate a lot of refactoring!  I've discarded a CMS solution in favor of an approach closer "to the iron"; CI seems the perfect fit.  Laravel is intriguing but exhausting so...

• What is the long view about ongoing support for CI 3.x branch once CI 4.x comes online?
  
• How compatible (in general) will v4.x be to 3.x in a PHP7 server environment? Migration issues?
  
• Is general recommendation to start with the less formative 4.0 hoping for production release in mid '18 or stick with 3.x branch and migrate the code when 3.x hits end of life?
  

TIA 
--Wolf--