A Concerned Grandpa
Although she appears to have forgotten last year's terror (kids do not forget such initial horrors) and shows excitement for this coming Halloween, your concerns are valid.
Four-years-old is mature enough to ask her if she wants to greet kids at the door this year. You may remind her that she was frightened last year and that you want her to have a good time with you this year. Take her at her word if she says she doesn't think she'll be scared of some of the costumes and makeup. Don't push her to do this, but offer to take her to a store that sells scary masks and costumes. You and she might try on some of the scarier masks and pretend to frighten each other. This may give you a clue as to her readiness.
I would have activities planned for her other than her answering the door with you (baking and decorating Halloween cookies, "pin the tail on the ghost", a Halloween piñata, telling spooky stories, bobbing for apples). You may tell her that you'll call on her to bring you candy for some of the kids at the door. That way you can screen the truly gruesome and invite her to join you for only the non-terrifying variety.
She can enjoy these holiday activities and look forward to them as Halloween rituals she shares with you every year.
She should never be made to feel ashamed for being scared. Always tell her that you understand that she's frightened and that you can sure see why. Assure her that these ghouls would scare you if you were a little boy her age. Then shift the focus to all the great, fun-filled activities for her. Make her feel that she's the special Halloween girl on this unique evening.
Have a great time grandpa. Make some memories with this darling little girl.