how to design a multiplier digital circuit using 4bit adders with output in BCD?

I want to design a circuit that multiplies a number by 6 an input binary number using only 4bit full adders. I designed the combinational logic but couldn't do it with 4bit adders as i were asked in my exam. any help?

• What is the input width? You might want to start by figuring out which bits in the output depend on which bits in the input. – Chris Stratton May 28 '16 at 1:56
• the input is only one digit (4 bits) – iMohaned May 28 '16 at 1:57
• I have considered the 6(0110) as the multiplier and the input(wxyz) as the multiplicand and made the mathematical operation with my hand and I have found that we need to add two numbers (wxy+wxyz) if i implement that with BCD adder , will I get the right answer? – iMohaned May 28 '16 at 2:04

6 in binary is 110 which means $1\times4 + 1\times2 + 0\times1 = 4 + 2$. So how could you calculate $6\times a$ simply?
So how do you calculation $6\times a$ using only adders?