If you have to have 30 guests, the village hall suggestion is ideal. But, at the risk of backlash, may I just ask why a 4 year old has to have 30 guests.
If you had her siblings and a few closer friends as opposed to the many acquaintances kids have at that age, with a tea party (picnic rug in living room/garden) and traditional games, I'm sure she would feel just as special and you would be less stressed.
I'm sure we end up inviting other kids half the time because we know the mums and feel we ought to or we feel it's manadatory to invite someone who's party we've been to and I don;t feel this is the case. Half a dozen close friends gives you more energy and time to give a special party for your little one as opposed to stretching yourself silly for other people.