How to resolve solver failure in constraints?

Question :  My code is mentioned below. when i run simulation it shows solver failure.

program automatic randomization_error_for_rand_mode_off;

  
bit clk = 1'b0;

 class randomize_with_io;
    rand bit [3:0] a = 0;
    rand bit [3:0] b = 0;
          int c = 0;

        /* Constraints with implication operator */
constraint on_a { 
                          a inside {[0:5]};
                          b inside {[0:15]};
                        }

  endclass: randomize_with_io

 initial begin
   randomize_with_io r;
   r = new();

   forever@(posedge clk)
     begin
     /* randomize() */
     r.a.rand_mode(0); //disabling the rand mode
     r.randomize() with {
                         (b inside {[0:15]}) -> a == 4;  
                        };

     $display("----------------------------------------"); 
     $display("|at time = %0t | a = %0d | b = %0d | c = %0d |", $time,r.a,r.b,r.c);
   end
     $display("----------------------------------------"); 
 end

initial begin 
  forever #5 clk = ~clk;
end

initial begin
#500 $finish;
end


endprogram : randomization_error_for_rand_mode_off

I am seeing following solver failure while running the simulation , how can i resolved it.

==================================================
Solver failed when solving following set of constraints
rand bit[3:0] a = 4'h0; // rand_mode = OFF
rand bit[3:0] b;    // rand_mode = ON 
constraint WITH_CONSTRAINT    // (from this) (constraint_mode = ON) (randomization_error_for_rand_mode_off.sv:28)
{
   (b inside {[0:15]}) -> (a == 4'h4);
}
==================================================