I can't boot OpenBSD using Grub2

[alessandro kirk]

I have the follow problem, on my secondary pata hdd, i have installed OpenBSD 
but Grub2 from Ubuntu 10.04 don't see it.
While if i start the hdd directly using the Bios, OpenBSD boot and work.
$ sudo fdisk -l /dev/sdb
Disco /dev/sdb: 6488 MB, 6488294400 byte
255 testine, 63 settori/tracce, 788 cilindri
Unità = cilindri di 16065 * 512 = 8225280 byte
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Identificativo disco: 0x00000000

Dispositivo Boot      Start         End      Blocks   Id  System
/dev/sdb4   *           1         788     6329578   a6  OpenBSD

I already tried many combination without success.
Naturally i made an:
#update-grub 
each time.

/etc/grub.d/40_custom
1)
#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
menuentry "OpenBSD" { 
set root=(hd1,4)
chainloader +1 
}

2)
set root=(hd1,4,a)
chainloader +1

3)
set root='(hd1,4)'
chainloader +1

4)
set root=(hd1,4)
chainloader +1
boot 

How resolve that problem?