XF-4 = FS: 34259
XF-26 = FS: 34128
XF-51 doesn't have an FS equivilant, but its Humbrol 108, which is an enamel. (Some charts put FS: 34087 olive drab as the equivilant to the humbrol, but I don't know for sure.)
For future reference, check out these sites:
http://www.hotel.wineasy.se/ipms/stuff_eng_colorcharts_tamiya.htm
http://www.ipmsstockholm.org/colorcharts/stuff_eng_colorcharts_tamiya.htm
http://www.angelfire.com/ca6/modelci/hints/color/tamiya.htm
Semper Fi