I'm sure you probally already know this but my girls seem to "pant" when hot and thirsty.
House her seperately and give her some sugar in her water for a quick boost. Just for 1/2 a day or she'll get the runs. Give her some kitchen scraps to see if she has really warded off food.
Tumors? Unless you get an MRI who knows!

overheating, they pant . gapeworm they gape. have you wormed them at all?

I was thinking she was hot, too. The first time my girls did it I was positive they had Gapeworm. I was thrilled to think they were just "sweating", as there's very few things that cure Gapeworm. I think a four year old chicken is past her real egg laying time. She's just takin' it easy.
Let us know how it turns out.
Carla

At four years she is just beginning to get past her prime as far as egg laying, so she may indeed be simply coming to the natural end of her egg laying period. Also it has been very hot and humid here (I'm near Los Angeles).

What breed is she? If she is a heavy breed, they don't fare so well in the heat. Try bringing her inside for a bit and keeping her cool and hydrated and see if the panting stops. Another possibility is that she is egg bound--if this is the case you have to move fast or she could die. Click on my 'My Page' link for more chicken info, including how to recognize and help an egg bound hen and how to recognize and treat for gapeworms and other parasites, and helping chickens beat the heat. There is also info there on how to handfeed her to assist her through an illness if needed.

Any other info you could give us on her would help...good luck! :)

Velvet