why if we say birchas hagomel only in these 4 situations do we say tefilas haderech when we go on a trip that doesnt involve ever layam or midbarios?
Your question points to a fundamental difference between Birkas ha'Gomel and Tefilas ha'Derech. One is a blessing for a miraculous deliverance that one experienced by the hands of Hash-m when one's life was in jeopardy (Birkas ha'Gomel). The other is a general request that one's travels be safe, free from all undesirable occurences (such as robbery), and not only life-threatening situations.
E. Chrysler