Behavioural - `Non-Synthesisable' RTL

Not all RTL is officially synthesisable, as defined by language standards.

RTL with event control in the body of a thread defines a state machine.

This state machine requires a program counter (PC) register at runtime (implied):

   input clk, din;
   output req [3:0] q;

   always begin
         q <= 1; 
         @(posedge clk) q <= 2;
         if (din) @(posedge clk) q <= 3;
         q <= 4;
         end

How many bits of PC are needed ? Is conditional event control synthesisable ? Does the output `q' ever take on the value 4 ?