16 in 4 hours should be fine, I've done similar parties before! (Though would recommend you ask them if it#s ok to go in 15 minutes early to set up and leave 15 minutes later to clear away! Make sure they don't include this in the 4 hours!). When I do parties like this I offer;
Mini Manicure / Pedicure (File, mini massage, polish)
Mini Back Massage
Reflexology Taster Session
Eyebrow Threading
Mini Facial (Cleanse, tone, massage, moisturise)
Aromatherapy Facial Massage
It's also a good idea to let the host know you'll be working flat out, so a little organisation on behalf of the guests is appreciated! Usually groups work out their own order, and the last one sends in the next one for me! I'm happy to work this way, usually it means my treatments are all mixed up (Which I prefer), rather than 2 hours of back massages followed by 2 hours of nails!
Good luck, and enjoy it; hard work, but fun