3/4GB memory on Precision Workstation 370 with RHEL4

Recently I add 2GB memory to my Dell Precision Workstation 370 system (Pentium4 Extreme Edition, 3.4GHz, Hyperthreading Enabled). This system was orignally configured with 2GB memory and installed with Redhat Enterprise Linux (RHEL) 3. Then I upgraded its OS to RHEL4 Update 2 and its BIOS to the latest A06 version. After plugging all 4GB memory into all four slots of the computer, a problem showed up:

Although all 4GB memory is correctly recognized in BIOS, only 3GB memory is shown in Linux:

[shrek@integrin Dell]$ free
total used free shared buffers cached
Mem: 3113776 680704 2433072 0 143292 406348
-/+ buffers/cache: 131064 2982712
Swap: 2096472 0 2096472

[shrek@integrin Dell]$ cat /proc/meminfo
MemTotal: 3113776 kB
MemFree: 2433136 kB
Buffers: 143304 kB
Cached: 406336 kB
SwapCached: 0 kB
Active: 185480 kB
Inactive: 402332 kB
HighTotal: 2226728 kB
HighFree: 2031552 kB
LowTotal: 887048 kB
LowFree: 401584 kB
SwapTotal: 2096472 kB
SwapFree: 2096472 kB
Dirty: 0 kB
Writeback: 0 kB
Mapped: 59404 kB
Slab: 72884 kB
Committed_AS: 181280 kB
PageTables: 3144 kB
VmallocTotal: 106488 kB
VmallocUsed: 42100 kB
VmallocChunk: 61940 kB
HugePages_Total: 0
HugePages_Free: 0
Hugepagesize: 2048 kB

The first few lines of the output of dmesg are:
BIOS-provided physical RAM map:
BIOS-e820: 0000000000000000 - 00000000000a0000 (usable)
BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
BIOS-e820: 0000000000100000 - 00000000bfe8ac00 (usable)
BIOS-e820: 00000000bfe8ac00 - 00000000bfe8cc00 (ACPI NVS)
BIOS-e820: 00000000bfe8cc00 - 00000000bfe8ec00 (ACPI data)
BIOS-e820: 00000000bfe8ec00 - 00000000c0000000 (reserved)
BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved)
BIOS-e820: 00000000fec00000 - 00000000fed00400 (reserved)
BIOS-e820: 00000000fed20000 - 00000000feda0000 (reserved)
BIOS-e820: 00000000fee00000 - 00000000fef00000 (reserved)
BIOS-e820: 00000000ffb00000 - 0000000100000000 (reserved)
2174MB HIGHMEM available.
896MB LOWMEM available.

I have searched on internet and am aware of the issues of 1GB-3GB kernel-user VM space split versus the 4GB-4GB one. So I tried both the standard SMP kernel and the hugemem kernel from RHEL4: kernel-smp-2.6.9-22.EL and kernel-hugemem-2.6.9-22.EL, both of which enables CONFIG_HIMEM and CONFIG_HIGHMEM64G options and the latter of which enables the 4GB-4GB kernel-user VM space option during kernel compilation. However the final results are similarly frustrating: both kernels only recognize 3GB memory.

So my task is to find a way to make my computer or OS recognize all 4GB memory on board. Any experts have any clues on this mystery? Many thanks in advance!