Using EA compiled with Build 765 in MT4 Build less than 765

SW can be backwards compatible, but it is extremely unlikely that it is forward compatible in all aspects. It's likely either the EA or the terminal would crash.

1. If Metaquotes checks for that, it may not load.
2. You can always check:

   int OnInit(){
      int     build = TerminalInfoInteger(TERMINAL_BUILD)
      if(build < __MQLBUILD__){
         PrintFormat("Terminal=%i, must be at least %i", build, __MQLBUILD__);
         return INIT_FAILED;
      }
      :

   Predefined Macro Substitutions - MQL4 Documentation