always_ff @(posedge clk) begin
  if (!rst_n) begin
    in_idx <= '0;  // input pointer
    out_idx <= '0; // output pointer
    bitcnt <= '0;  // number of stored bits
    out_vld <= 1'b0;
  end
  else begin
    bitcnt <= next_bitcnt;
    if (in_vld) begin
      store[ in_idx +: 6] <= in;
      in_idx <= (in_idx + 6) % STORE_BITS;
    end
    out_vld <= (bitcnt >= 4 || in_vld);
    if (out_vld) begin
      out_idx <= (out_idx + 4) % STORE_BITS;
    end
  end
end

always_comb begin
  next_bitcnt = bitcnt;
  if (in_vld) begin
    next_bitcnt += 6;
  end
  if (bitcnt >= 4 || in_vld) begin
    next_bitcnt -= 4;
  end
end

assign out = store[ out_idx +: 4 ];