lesspipe.sh doesn't save tabstop setting

Question

I like to use 4 spaces for tabstop, so my less settings is:

export LESS="-x4 -FRS"
export LESSOPEN='|lesspipe.sh %s'

But when I view source file with less src.c I see 8 spaces which is not what I want. When I view file without lesspipe.sh like this cat src.c | less or without highlighting less src.c: it looks just fine. How to overcome this behaviour?