We don't usually do big parties, so inviting 4ish kids sounds about perfect to me. I read one thing about inviting as many kids as the child's age, as a general rule. But its really whatever works for you.
Are you trying to get a group discount for the trampoline place?
If not, then maybe you could try to even out the cost between the two options and let her choose (if both options are ok with you). Like she can invite 4 friends to the trampoline place, or more friends (set a number) to a party at your house.
My oldest is turning 6 in June. She wants a unicorn themed birthday. We are just going to have it at home. My DH is working on building a cool new 2 story clubhouse/swing set which will be done by then, so the kids can play on that and the climbing dome, and I will probably put a sprinkler out. We will have a unicorn cake and a few balloons... probably pizza, "unicorn poop" cookies and "unicorn horns" made of frosting on ice cream cones and sprinkles.
She has about 5 friends that she will invite, but some of those have siblings that are friends with my middle child, so they will come, too. Plus parents. For me, that already feels like a ton of people. lol