Konri,
In your 46xxsetings.txt file, do you lock the ethernet ports to 100MB/Full? Do you have the ethernet POE switch set to lock the ports at 100MB/Full? We always do both, its just more reliable.
Here is a sample of a 46xxsettings file I use on an S8300C, we use the S8300C for both TFTP and HTTP/HTTPS for both 4600 and 9600 series phones.
your option 242 string on the DHCP server needs to look like this:
Option 242: MCIPADD=192.168.1.245,MCPORT-1719,TLSSERVER=192.168.1.245
The above IP's are an example, you MUST use TLSSERVER NOT HTTPSRVR, in the Option 242 string. You also must have a SET HTTPSRVR 192.168.1.245 [example S8300] in the 46xxsettings.txt file for the S8300 to serve out the HTTP/HTTPS files properly. Just copy your 4600 files and 9600 files to your /tftpboot directory. There are also some .tar.gz files you can install that set up the /tftpboot directory for you, available on the AVAYA website for the 9600 R1.5 and 4600 R1.8.3 firmware, mostly for CM 4.0
SET AUTH 0
SET HTTPSRVR 192.168.1.245 [this is the IP of the S8300]
SET STATIC 0
SET AGCHAND 1
SET AGCHEAD 1
SET AGCSPKR 1
SET L2Q 2
SET VLANTEST 60
SET DHCPSTD 0
SET PHY1STAT 5
SET PHY2STAT 5
SET PROCSTAT 0
SET PROCPSWD 27238
SET SNMPSTRING private
SET ICMPDU 1
SET ICMPRED 0
SET IRSTAT 0
SET APPSTAT 1
SET OPSTAT 111
SET PHNDPLENGTH 4
SET PHNLDLENGTH 10
SET PHNOL 9
SET PHNLD 1
SET PHNIC 11
SET PHNCC 1
SET SYSLANG English
SET RESTORESTAT 1
SET FTPUSERSTAT 1
SET AUDIOENV 0
SET AUDIOSTHD 0
SET AUDIOSTHS 0
SET ENHDIALSTAT 1
SET VLANSEP 0
SET BAKLIGHTOFF 120
SET RESTORESTAT 1
## SET FTPSRVR 192.168.1.245
## SET BRURI
SET LANGSYS English
SET LANG0STAT 1
SET LANG1FILE mlf_S15_v14_russian.txt
SET LANG2FILE mlf_S15_v14_spanish.txt
SET LANG3FILE mlf_S15_v14_french_paris.txt
SET LANG4FILE mlf_S15_v14_german.txt
SET MSGNUM 3399
## SET MCIPADD 192.168.1.245
SET REREGISTER 20
SET CTISTAT 0
SET CTIUDPPORT 49721
SET UNNAMEDSTAT 1
SET L2QAUD 6
SET L2QSIG 6
Mitch