I have a number of ISR 4431s that all droop in the rack, no matter how tightly I do up the rack bolts. This means that they intrude significantly into the space underneath them preventing other kit being mounted there. (this may even be due to the rack rails bending) I thought that there used to be a kit to allow 4-post mounting similar to that used on 1U Nexus switches. Has this kit been removed from sale?
I'm not aware of there ever being a four post kit for this router. I've run into similar issues with many of the switches also.
If possible, you can flip the bracket so that the router sticks out about an inch or so from the front of the rack which may provide better alignment, or purchase an additional bracket and mount it front and back provided the rack rails can accommodate.
Unfortunately, the constraints of the rack don't allow me to set the mounting lugs anywhere other than level with the front plane of the router. It seem pretty poor design that when installed like this the router hangs at around 10-15 degrees to the horizontal. The Nexus solution is pretty elegant and flexible.
But while thinking about it a thought came to me and I don't know when I'll get the chance to try it out, but I'll pass it on to you. What about putting a washer between the bracket and rack only on the top screws. It might just provide enough of an offset to mitigate or minimize the problem.
Another possible solution is to put a cable management bracket under the router and on the back of a two post rack. It can provide additional support.