The problem with moving her to another tank or a breeding net is that it may stress her so much that she aborts the fry and then you end up with none!
The best way is to let her have the fry in her tank and then scoop them up and put the fry in the breeding net.
I can't believe that you have no means of collecting the fry, surely you have a fish net, if you haven't, go buy one, they're cheap enough.
It's not a given that all the fry will get eaten, yes you'll loose some, but not all of them, they're very good at hiding and very quick swimmers. Nice bushy plants like ambulia or water sprite make an excellent fry hideout as do floating plants, rocks, shells and pots. Just put some panty hose/tights over your filter intake so that they can't get sucked into it.
Also, given that you have 4 female platy's you're going to be up to your eyeballs in fry before you know it!