Group By with condition

Let´s suppose I have the following query:

SELECT fld1, fld2, fld3, fld4 FROM tblTable1

INNER JOIN ............

GROUP BY fld1, fld2, fld3, fld4

What I need is to group by "fld4" if and only if its value is "TL". If its value is different than "TL" then no grouping should be performed on this field.

I will very much appreciate ypur feedback.

Respectfully,

Jorge Maldonado