There are eight computers and four printers.One set contains 8 computers $C_1,C_2...C_8$and one set contains 4 printers $P_1,P_2,P_3,P_4$.It is given that four computers $(C_1,C_2,C_3,C_4)$ can directly access 4 printers.It means $(C_1,C_2,C_3,C_4)$ should be connect to the $(P_1,P_2,P_3,P_4)$.
Total cables required to connect 4 computers to 4 different printers = $4\times 4=16$
Now remaining 4 computers $(C_5, C_6, C_7, C_8)$ can connect with a minimum number of cables to the printer which is 1.For each remaining computers, one cable required to connect printers.
Hence, Least number of cables required to connect 8 computers to 4 printers=16+4=20