configure then make, gives:

gcc -DHAVE_CONFIG_H -I. -I/u01/root/source/php4/ext/mcrypt -I../.. -I../../Zend -I/u01/root/source/php4 -I/root/source/apache//src/include -I/root/source/apache//src/os/unix -I/usr/local/include -I/u01/root/source/php4/ext/mysql/libmysql -I/u01/app/oracle/product/8.0.5/rdbms/demo -I/u01/app/oracle/product/8.0.5/network/public -I/u01/app/oracle/product/8.0.5/plsql/public -I/u01/app/oracle/product/8.0.5/rdbms/demo -I/u01/app/oracle/product/8.0.5/network/public -I/u01/app/oracle/product/8.0.5/plsql/public -I/u01/root/source/php4/ext/xml/expat/xmltok -I/u01/root/source/php4/ext/xml/expat/xmlparse  -DXML_BYTE_ORDER=12 -D_REENTRANT -g -O2 -Wall  -c mcrypt.c && touch mcrypt.lo
mcrypt.c: In function `php_minit_mcrypt':
mcrypt.c:132: `TRIPLEDES' undeclared (first use in this function)
mcrypt.c:132: (Each undeclared identifier is reported only once
mcrypt.c:132: for each function it appears in.)
mcrypt.c:157: `MCRYPT_RC6_256' undeclared (first use in this function)
mcrypt.c:159: `MCRYPT_RC6_128' undeclared (first use in this function)
mcrypt.c:160: `MCRYPT_RC6_192' undeclared (first use in this function)
mcrypt.c: In function `php_if_mcrypt_ofb':
mcrypt.c:290: warning: implicit declaration of function `init_mcrypt_ofb'
mcrypt.c:292: warning: implicit declaration of function `mcrypt_ofb'
mcrypt.c:292: warning: implicit declaration of function `mdecrypt_ofb'
mcrypt.c:292: warning: implicit declaration of function `end_mcrypt_ofb'
mcrypt.c: In function `php_if_mcrypt_cfb':
mcrypt.c:311: warning: implicit declaration of function `init_mcrypt_cfb'
mcrypt.c:313: warning: implicit declaration of function `mcrypt_cfb'
mcrypt.c:313: warning: implicit declaration of function `mdecrypt_cfb'
mcrypt.c:313: warning: implicit declaration of function `end_mcrypt_cfb'
mcrypt.c: In function `php_if_mcrypt_cbc':
mcrypt.c:336: warning: implicit declaration of function `init_mcrypt_cbc'
mcrypt.c:340: warning: implicit declaration of function `mcrypt'
mcrypt.c:343: warning: implicit declaration of function `mcrypt_cbc'
mcrypt.c:343: warning: implicit declaration of function `mdecrypt_cbc'
mcrypt.c:343: warning: implicit declaration of function `end_mcrypt_cbc'
mcrypt.c: In function `php_if_mcrypt_ecb':
mcrypt.c:361: warning: implicit declaration of function `init_mcrypt_ecb'
mcrypt.c:363: warning: implicit declaration of function `mcrypt_ecb'
mcrypt.c:363: warning: implicit declaration of function `mdecrypt_ecb'
mcrypt.c:363: warning: implicit declaration of function `end_mcrypt_ecb'
make[3]: *** [mcrypt.lo] Error 1
make[3]: Leaving directory `/u01/root/source/php4/ext/mcrypt'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/u01/root/source/php4/ext/mcrypt'
make[1]: *** [all] Error 1
make[1]: Leaving directory `/u01/root/source/php4/ext'
make: *** [all-recursive] Error 1

Ack ack, what saddness. Worked in B3 :)
Standard mcrypt RPMs installed.