Setting shared_buffers in postgresql.conf does not seem to take effect

We’re on CentOS release 6.6, PostgreSQL version 8.4.20. (Yes, this is not bleeding edge.)

In postgresql.conf, we have:

shared_buffers = 4096MB

kernel shm values are set nice and high:

[root@green data]# sysctl -a | grep shm
kernel.shmmax = 15922077696
kernel.shmall = 3887226
kernel.shmmni = 4096
kernel.shm_rmid_forced = 0

We have plenty of memory:

[root@green data]# free
             total       used       free     shared    buffers     cached
Mem:      31097812   30474972     622840    2873672    1961088   20565360
-/+ buffers/cache:    7948524   23149288
Swap:      1959920      93852    1866068

Yet the value for shared_buffers reported by pg_settings is only 512MB, not the 4GB set in postgresql.conf:

postgres=# select name, setting, min_val, max_val, context from  
pg_settings where name='shared_buffers';
      name      | setting | min_val |  max_val   |  context   
----------------+---------+---------+------------+------------
 shared_buffers | 524288  | 16      | 1073741823 | postmaster

Yes, we’ve done a full restart, and SHOW config_file confirms that I’ve edited the right postgresql.conf.

My very great thanks to anyone who can provide insight into this mystery.