Asynchronous Down Counter using D Flip Flops
What I found is, your code is correct but at posedge count down is happen but at same tick 0xF is (reset value), also driven by this code, check that monitor output, see attached log file

module DFlipFlop
(
  input wire    reset_n,
  input wire    clk,
  input wire    d,
  output wire   q,
  output wire   q_n
);

  wire w1, w2, w3, w4, w5, w6;


 //master
 nand na1(w1, d, ~clk);
 nand na2(w2, ~clk, ~d);
 nand na3(w3, reset_n, w1, w4);
 nand na4(w4, w3, w2);

 //slave
 nand na5(w5, w3, clk);
 nand na6(w6, clk, ~w3);
 nand na7(q, reset_n, w5, q_n);
 nand na8(q_n, q, w6);

endmodule


module Count4Down 
(
  input wire      reset_n,
  input wire      clk,
  output [3:0]    q
);

  wire qn0, qn1, qn2, qn3;

  DFlipFlop ff0(reset_n, clk, qn0, q[0], qn0);
  DFlipFlop ff1(reset_n, q[0], qn1, q[1], qn1);  
  DFlipFlop ff2(reset_n, q[1], qn2, q[2], qn2);
  DFlipFlop ff3(reset_n, q[2], qn3, q[3], qn3);

endmodule

module testbench; 
  reg clk, reset_n; 
  wire [3:0] q; 

  Count4Down u1(.clk(clk), .reset_n(reset_n), .q(q)); 


  initial
  begin
    $display ("Clk\t reset_n\t count\t ");
    $monitor ("%b\t %b\t\t %b",clk,reset_n,q);
    clk = 0;
    reset_n = 0;
    #10 reset_n = 1;
    #100 reset_n = 0;
    #20 $finish;
  end

  always clk = #1 ~clk;
endmodule


I think is q is have complicated driving logic, check it. 

QuestaSim-64 qverilog 10.4 Compiler 2014.12 Dec  2 2014
Start time: 15:59:44 on Mar 22,2016
qverilog count.v 
-- Compiling module DFlipFlop
-- Compiling module Count4Down
-- Compiling module testbench

Top level modules:
    testbench
Reading pref.tcl

 10.4

 vsim -lib work testbench -c -do "run -all; quit -f" -appendlog -l qverilog.log -vopt 
 ** Note: (vsim-8009) Loading existing optimized design _opt
 //  Questa Sim-64
 //  Version 10.4 linux_x86_64 Dec  2 2014
 //
 //  Copyright 1991-2014 Mentor Graphics Corporation
 //  All Rights Reserved.
 //
 //  THIS WORK CONTAINS TRADE SECRET AND PROPRIETARY INFORMATION
 //  WHICH IS THE PROPERTY OF MENTOR GRAPHICS CORPORATION OR ITS
 //  LICENSORS AND IS SUBJECT TO LICENSE TERMS.
 //  THIS DOCUMENT CONTAINS TRADE SECRETS AND COMMERCIAL OR FINANCIAL
 //  INFORMATION THAT ARE PRIVILEGED, CONFIDENTIAL, AND EXEMPT FROM
 //  DISCLOSURE UNDER THE FREEDOM OF INFORMATION ACT, 5 U.S.C. SECTION 552.
 //  FURTHERMORE, THIS INFORMATION IS PROHIBITED FROM DISCLOSURE UNDER
 //  THE TRADE SECRETS ACT, 18 U.S.C. SECTION 1905.
 //
 Loading work.testbench(fast)
 run -all
 Clk     reset_n     count   
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   1       1111
 1   1       1110
 0   1       1110
 1   1       1101
 0   1       1101
 1   1       1100
 0   1       1100
 1   1       1011
 0   1       1011
 1   1       1010
 0   1       1010
 1   1       1001
 0   1       1001
 1   1       1000
 0   1       1000
 1   1       0111
 0   1       0111
 1   1       0110
 0   1       0110
 1   1       0101
 0   1       0101
 1   1       0100
 0   1       0100
 1   1       0011
 0   1       0011
 1   1       0010
 0   1       0010
 1   1       0001
 0   1       0001
 1   1       0000
 0   1       0000
 1   1       1111
 0   1       1111
 1   1       1110
 0   1       1110
 1   1       1101
 0   1       1101
 1   1       1100
 0   1       1100
 1   1       1011
 0   1       1011
 1   1       1010
 0   1       1010
 1   1       1001
 0   1       1001
 1   1       1000
 0   1       1000
 1   1       0111
 0   1       0111
 1   1       0110
 0   1       0110
 1   1       0101
 0   1       0101
 1   1       0100
 0   1       0100
 1   1       0011
 0   1       0011
 1   1       0010
 0   1       0010
 1   1       0001
 0   1       0001
 1   1       0000
 0   1       0000
 1   1       1111
 0   1       1111
 1   1       1110
 0   1       1110
 1   1       1101
 0   1       1101
 1   1       1100
 0   1       1100
 1   1       1011
 0   1       1011
 1   1       1010
 0   1       1010
 1   1       1001
 0   1       1001
 1   1       1000
 0   1       1000
 1   1       0111
 0   1       0111
 1   1       0110
 0   1       0110
 1   1       0101
 0   1       0101
 1   1       0100
 0   1       0100
 1   1       0011
 0   1       0011
 1   1       0010
 0   1       0010
 1   1       0001
 0   1       0001
 1   1       0000
 0   1       0000
 1   1       1111
 0   1       1111
 1   1       1110
 0   1       1110
 1   1       1101
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1112
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 0   0       1111
 1   0       1111
 ** Note: $finish    : count.v(59)
    Time: 130 ns  Iteration: 0  Instance: /testbench
 End time: 15:59:45 on Mar 22,2016, Elapsed time: 0:00:01
 Errors: 0, Warnings: 0