but shorting 4 &6 7 & 8 are not at all required.. Just shorting pins 2 & 3 should give you loop back result as you are going to use NULL modem.. so there is no question of any other pins other than pins 2,3,5.
You should get the output of loopback only with shorting pin 2 & 3.
if you get the output then your modem will also work ...
connect modem and test again.. if the output does not come then change the tx and rx pin connection at one end of cable and try...
but your loop back test is not working if you try to short other pins other than pin 2,& 3.