Here is the answers, well done guys and gals
4-letter words
alee, anil, elan, ilea, lain, lane, lean, leap, nail, nape, neap, nipa, pail, pain, pale, palp, pane, peal, pean, pial, pian, pina, plan, plea,
5-letter words
alien, aline, anele, anile, appel, apple, elain, lapin, liane, nappe, palpi, panel, pappi, penal, pepla, pipal, plain, plane, plena
6-letter words
alpine, nappie, penial, pineal
7-letter words
elapine, panpipe
9-letter words
pineapple

ok I won't