Difference between program and module block

1. A program block can not instantiate a module block. On the opposite side, a module block can instantiate another module or program block.
2. A program block can not have an interface, user-defined primitives (UDP), always block or nested program.
3. The initial block inside the program block is scheduled in the reactive region whereas the initial blocks inside the module lock are scheduled in the active region.

// Code your testbench here
// or browse Examples
program tb_pro(input [3:0] out);
  initial begin
    if(out == 2)
      $display("Design assigned out is 2");
    else
      $display("Design assigned out = %0d", out);
  end
endprogram

module dut_example (output bit [3:0] out);
  initial begin
    out <= 2;
  end
endmodule

module tb_mod_top;
  wire [3:0] out;
  
  dut_example DUT(out);
  tb_pro tb(out);
endmodule

Output:

Design assigned out is 2