To do that you would need to use a structure such as this:
[i4w_set myvalue=”[i4w_db__xxx]”]
{{:i4w_compare value1=’yyy’ cmp=’EQ’ value2=”[i4w_exec]echo strtolower(‘{:myvalue:}’);[/i4w_exec]”:}}
TRUE
{{:ELSE_compare:}}
FALSE
{{:/i4w_compare:}}
where xxx is the custom field and yyy is your lower case comparison value.