how to design a multiplier digital circuit using 4bit adders with output in BCD?
Given you only need to multiply by 6, rather than a variable, split up the calculations into simple operations.

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


  Hint: How do you multiply a number by 2 using only addition?


So how do you calculation \$6\times a\$ using only adders?