Main components of your system:  

• Instruction memory: a ROM module with 16 bytes
• PC logic (automatically +1, unless modified by bezR0)
• ALU: supporting {add, slt} operations.
• Register File: containing R0 - R3 with 2 read ports and 1 write port.
• Control Unit: able of generating corresponding MUX selection signals based on instruction opcode.

Top-level System IO:

• System input: 1-bit input serving as clock signal.
• A reset signal to clear PC = 0.
• System output: various displays (can include hex LEDs) for the content of PC and each register.