You are an expert Verilog hardware designer working on a complete FPGA or ASIC project. You are given the full source code of a Verilog project. One of the modules in the project is incomplete — its implementation is missing and marked with:

// >>> Module Implementation Begin

// <<< Module Implementation End

Your task is to complete the missing implementation so that the entire project functions correctly. You must infer what the missing module or logic is supposed to do by analyzing the rest of the codebase: module names, signal connections, instantiations, comments, and usage patterns.

Follow these rules:

1. Only generate Verilog code to go between the `// >>> Module Implementation Begin` and `// <<< Module Implementation End` comments.
2. Use Verilog-2001 syntax. Ensure the code is synthesizable.
3. Assume the goal is a working, integrated hardware design — your code must make the whole system function as intended.
4. Analyze the naming, I/O ports, and how the module is instantiated or used in other parts of the project.
5. Implement correct behavior based on design context. Do not guess randomly — reason it out.
6. Avoid vendor-specific features unless explicitly required.
7. Do not output comments, explanation, or anything outside the required code region unless explicitly asked.

Your output should contain **only the Verilog code** that fits inside the marked region.

module tt_um_TD4_Assy_KosugiSubaru (
    input  wire [7:0] ui_in,    // Dedicated inputs
    output wire [7:0] uo_out,   // Dedicated outputs
    input  wire [7:0] uio_in,   // IOs: Input path
    output wire [7:0] uio_out,  // IOs: Output path
    output wire [7:0] uio_oe,   // IOs: Enable path (active high: 0=input, 1=output)
    input  wire       ena,      // will go high when the design is enabled
    input  wire       clk,      // clock
    input  wire       rst_n     // reset_n - low to reset
);

wire [7:0] data;
wire [3:0] port_i, port_o, addres, ALU_to_reg, sel_to_ALU, regA_to_sel, regB_to_sel, load, op, im;
wire [1:0] select;
wire carry, cf, co_from_pc;

reg [3:0] uio_oe_reg;

assign uio_oe [4] = 1'b0;
assign uio_out [4] = 1'b0;
assign uio_oe [7:5] = 3'b111;
assign uio_oe [3:0] = uio_oe_reg [3:0];

assign port_i [3:0] = uio_in [3:0];
assign uio_out [3:0] = port_o [3:0];

assign uo_out [3:0] = addres[3:0];
assign data [7:0] = ui_in [7:0];
assign op = data[7:4], im = data[3:0];

assign uo_out [4] = cf;
assign uo_out [7:5] = ALU_to_reg [2:0];
assign uio_out [5] = ALU_to_reg [3];
assign uio_out [7:6] = select [1:0];

always @(uio_in [4]) begin
    if (uio_in [4] == 1'b0) begin
        uio_oe_reg [3:0] <= 4'b0000;
    end else if (uio_in [4] == 1'b1) begin
        uio_oe_reg [3:0] <= 4'b1111;
    end
end

register_ff_4bit reg_A (.in(ALU_to_reg), .out(regA_to_sel), .ld(~load [0]), .clk(clk), .rst(rst_n));
register_ff_4bit reg_B (.in(ALU_to_reg), .out(regB_to_sel), .ld(~load[1]), .clk(clk), .rst(rst_n));
register_ff_4bit OUT   (.in(ALU_to_reg), .out(port_o), .ld(~load [2]), .clk(clk), .rst(rst_n));

ALU_adder_4bit ALU0    (.in_a(sel_to_ALU), .in_b(im), .out(ALU_to_reg), .ci(1'b0), .co(carry));

pc       pc0  (.in(ALU_to_reg), .out(addres), .ld(~load [3]), .clk(clk), .rst(rst_n), .co(co_from_pc));

decoder  dec0 (.op(op), .c_n(~cf), .s(select), .ld_n(load));

selector sel0 (.in_a(regA_to_sel), .in_b(regB_to_sel), .in_c(port_i), .in_d(4'b0000), .s(select), .out(sel_to_ALU));

ff_1bit  cf0  (.in(carry), .out(cf), .clk(clk), .rst(rst_n), .pr(1'b1));

endmodule


module register_ff_4bit(in, out, ld, clk, rst);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule



module ff_1bit(in, out, clk, rst, pr);
    input in;
    input clk, rst, pr;
    output out;

    reg out;
    always @(posedge clk or negedge rst) begin
        if (rst == 1'b0) begin
            out <= 0;
        end else begin
            if (pr == 1'b0) begin
                out <= 1;
            end else begin
                out <= in;
            end
        end
    end
endmodule

module pc(in, out, ld, clk, rst, co);
    input [3:0] in;
    input ld, clk, rst;
    output [3:0] out;
    output co;

    reg [3:0] out;
    reg co;
    always @(posedge clk or negedge rst) begin
        if (rst == 1'b0) begin
            out <= 4'b0000;
            co <= 1'b0;
        end else if (out == 4'b1111) begin
            co <= 1'b1;
        end else begin
            if (ld == 1'b1) begin
                out <= in;
            end else if (ld == 1'b0) begin
                out <= out + 4'b0001;
            end
        end
    end
endmodule

module ALU_adder_4bit(in_a, in_b, out, ci, co);
    input [3:0] in_a, in_b;
    input ci;
    output [3:0] out;
    output co;

    reg [3:0] out;
    reg co;
    always @(in_a or in_b or ci) begin
        {co, out} <= in_a + in_b + ci;
    end
endmodule

module decoder(op, c_n, s, ld_n);
    input [3:0] op;
    input c_n;
    output [1:0] s;
    output [3:0] ld_n;

    reg [1:0] s;
    reg [3:0] ld_n;
    always @(op or c_n) begin
        s [1] <= op[1];
        s [0] <= op[0] | op[3];
        ld_n[0] <= op[2] | op[3];
        ld_n[1] <= op[3] | ~(op[2] & 1 & 1);
        ld_n[2] <= ~(1 & ~(op[2] & 1 & 1) & op[3]);
        ld_n[3] <= ~(op[2] & op[3] & (op[0] | c_n));
    end
endmodule

module selector(in_a, in_b, in_c, in_d, s, out);
    input [3:0] in_a, in_b, in_c, in_d;
    input [1:0] s;
    output [3:0] out;

    reg [3:0] out; 
    always @(in_a or in_b or in_c or in_d or s) begin
        case (s)
            2'b00: out <= in_a;
            2'b01: out <= in_b;
            2'b10: out <= in_c;
            2'b11: out <= in_d;
            default: out <= 4'bxxxx;
        endcase
    end
endmodule


Reference solution:
rtl/generated.v:     input [3:0] in;
    input ld, clk, rst;
    output [3:0] out;

    reg [3:0] out;
    always @(posedge clk or negedge rst) begin
        if (rst ==1'b0) begin
            out <= 4'b0000;
        end else begin 
            if (ld == 1'b1) begin
                out <= in;
            end else if (ld == 1'b0) begin
                out <= out;
            end
        end
    end