Test script:
<?php
session_unregister('test');
echo "test";
?>

Red Hat Linux release 6.1 (Cartman)
Linux backup 2.2.12-20smp
Apache 1.2.23
php 4.1.2 

./configure  --with-apxs --with-mysql=/usr/local --enable-debug=yes --enable-trackvars --enable-sockets --enable-calendar --with-gd=../gd-1.8.4 --with-jpeg-dir=/usr/ --with-png-dir=/usr/ --with-imap=/usr/local/ --with-ldap --with-gettext --with-zlib


Program received signal SIGSEGV, Segmentation fault.
0x4020a544 in zend_hash_del_key_or_index (ht=0x81bcf24, arKey=0x81e5b6c "test", nKeyLength=5, h=0, flag=0) at zend_hash.c:484
484             p = ht->arBuckets[nIndex];
(gdb) bt
#0  0x4020a544 in zend_hash_del_key_or_index (ht=0x81bcf24, arKey=0x81e5b6c "test", nKeyLength=5, h=0, flag=0) at zend_hash.c:484
#1  0x40253358 in zif_session_unregister (ht=1, return_value=0x81e5b84, this_ptr=0x0, return_value_used=0) at session.c:1201
#2  0x401f7605 in execute (op_array=0x81e08dc) at ./zend_execute.c:1590
#3  0x4020636b in zend_execute_scripts (type=8, retval=0x0, file_count=3) at zend.c:814
#4  0x402137e3 in php_execute_script (primary_file=0xbffffa30) at main.c:1307
#5  0x4020fa21 in apache_php_module_main (r=0x81287fc, display_source_mode=0) at sapi_apache.c:90
#6  0x402104f3 in send_php (r=0x81287fc, display_source_mode=0, filename=0x0) at mod_php4.c:575
#7  0x40210554 in send_parsed_php (r=0x81287fc) at mod_php4.c:590
#8  0x806d823 in ap_invoke_handler ()
#9  0x80822b9 in process_request_internal ()
#10 0x808231c in ap_process_request ()
#11 0x807961e in child_main ()
#12 0x80797d0 in make_child ()
#13 0x8079929 in startup_children ()
#14 0x8079f86 in standalone_main ()
#15 0x807a753 in main ()
#16 0x400b51eb in __libc_start_main (main=0x807a3fc <main>, argc=2, argv=0xbffffd54, init=0x804f878 <_init>, fini=0x80b131c <_fini>, rtld_fini=0x4000a610 <_dl_fini>,
    stack_end=0xbffffd4c) at ../sysdeps/generic/libc-start.c:90
(gdb)


If you need further information i'll be happy to help!