I was not able to start jumpstart installation for my SF4800.
when tryng to boot from network, the system gave a "ILLEGAL INSTRUCTION" error, as shown below:

-------------------------------------------------

Sun Fire 4800
OpenFirmware version 5.20.10 (05/21/08 16:12)
Copyright 2008 Sun Microsystems, Inc.  All rights reserved.
Use is subject to license terms.
SmartFirmware, Copyright (C) 1996-2001.  All rights reserved.
36864 MB memory installed, Serial #50856896.
Ethernet address 0:3:ba:8:3:c0, Host ID: 830803c0.


Rebooting with command: boot net - install
/ssm@0,0/pci@18,700000/pci@1/network@0: 100 Mbps full duplex link up
Requesting Internet Address for 0:3:ba:f:6a:6
Requesting Internet Address for 0:3:ba:f:6a:6
Requesting Internet Address for 0:3:ba:f:6a:6
Requesting Internet Address for 0:3:ba:f:6a:6
Requesting Internet Address for 0:3:ba:f:6a:6
TL = 1, TT = 10. ERROR: Illegal Instruction
TSTATE= 0x1400 [ccr = 0x0, asi = 0x0, pstate = 0x14, cwp = 0x0]
TPC= 0000000000004000
TNPC= 0000000000004004
TICK= 800000485b00c3b0, TICKCMP = 8000000000000000

debugger entered.

--------------------------------------------------------

"illegal instruction" when happening during a boot,means that the system is trying to boot from a non-boot device, that the boot information has become corrupted, or that the boot information is meant for a different architecture.

Indeed I checked directory /tftpboot n jumpstart server oand I noticed that file inetboot.SUN4U.Solaris_10-4 was currupted. its size was very small and it was an ascii file.
While the correct format is the following:

sv213077-(root)-/tftpboot # ls -lrt inet*
-rw-r--r--   1 root     other     215184 Sep 19  2008 inetboot.SUN4U.Solaris_10-2
-rw-r--r--   1 root     other     161056 Sep 19  2008 inetboot.SUN4U.Solaris_8-2
-rw-r--r--   1 root     other     221544 May  3 11:17 inetboot.SUN4U.Solaris_10-4
-rw-r--r--   1 root     other     217016 May  3 11:17 inetboot.SUN4U.Solaris_10-3
sv213077-(root)-/tftpboot # file inetboot.SUN4U.Solaris_10-4
inetboot.SUN4U.Solaris_10-4:    ELF 64-bit MSB executable SPARCV9 Version 1, statically linked, stripped
sv213077-(root)-/tftpboot # file inetboot.SUN4U.Solaris_10-3
inetboot.SUN4U.Solaris_10-3:    ELF 64-bit MSB executable SPARCV9 Version 1, statically linked, stripped

After copying the correct files from another jumpstart server, the problem disappeared and installation was ok.
So in case you happen to spot the same error, check the size and format of these files.

Hope it helps.