RAID5 resize the number of activated Hard Disks

RAID5 Resize and activate the number of hard disks-general Linux technology-Linux technology and application information. The following is a detailed description. RAID5 started with three hard disks with the same capacity. The capacity should be the total capacity of the two hard disks.
A hard disk added with mdadm-G/dev/md0-n4
[Root @ localhost tmp] # mdadm-D/dev/md0
/Dev/md0:
Version: 00.90.03
Creation Time: Mon May 18 16:43:19 2009
Raid Level: raid5
Array Size: 3145536 (3.00 GiB 3.22 GB)
Device Size: 1048512 (1024.11 MiB 1073.68 MB)
Raid Devices: 4
Total Devices: 4
Preferred Minor: 0
Persistence: Superblock is persistent

Update Time: Mon May 18 17:43:37 2009
State: clean
Active Devices: 4
Working Devices: 4
Failed Devices: 0
Spare Devices: 0

Layout: left-lateral ric
Chunk Size: 64 K

UUID: b6c7646a: d2ef118f: 0f5aec86: bda29c7f
Events: 0.726

Number Major Minor RaidDevice State
0 8 0 0 active sync/dev/sda
1 8 16 1 active sync/dev/sdb
2 8 32 2 active sync/dev/sdc
3 8 48 3 active sync/dev/sdd
In addition, the capacity also increases the capacity of a hard disk, but how does the capacity not increase after mounting?
[Root @ localhost tmp] # df-h

/Dev/mapper/VolGroup00-LogVol00
6.7G 6.5G 0 100%/
/Dev/hda1 99 M 11 M 83 M 12%/boot
Tmpfs 506 M 0 506 M 0%/dev/shm
/Dev/md0 2.0G 33 M 2.0G 2%/tmp/raid

Please advise !!