Hello,I’m your friend.I have four letters.My first letter is in the word 'boot',but not in

Hello,I’m your friend.I have four letters.My first letter is in the word 'boot',but not in
'foot'.You can find my second and third letters in the word 'skirt',you also can find them in the word 'shirt'.My fourth letter is in the word 'date',but not in 'late'.What am