An expl3 solution. Here, I use the fact that \tl_map_function:nN will 'eat' spaces, and do proper case folding rather than lower casing (see the Unicode docs for the reasons that this is important):
\documentclass{article}\usepackage{expl3,xparse}\ExplSyntaxOn\NewExpandableDocumentCommand \compareStrings { m m +m +m } { \str_if_eq:eeTF % \str_if_eq_x:nnTF in older code { \tl_map_function:fN { \str_foldcase:n {#1} } \use:n } { \tl_map_function:fN { \str_foldcase:n {#2} } \use:n } {#3} {#4} }\cs_generate_variant:Nn \tl_map_function:nN { f }\ExplSyntaxOff\begin{document}\compareStrings{test}{tesT}{1}{2}\end{document}