Hm... what about an oven lock? Like this:
http://www.onestepahead.com/catalog/...mSource=Search
There's various types. Also search for refrigerator lock or appliance lock.
If you *really* can't find any means to physically secure the door, then you have to secure the child. You get a harness. You explain to Abigail (who is almost four, right?) that Mommy needs to keep her safe, and that means keeping her in the house when Mommy can't come outside with her. Since she can't follow that rule on her own, whenever Mommy needs to do something like go to the bathroom, Abigail puts on the harness, and Mommy keeps hold of her.
I'm serious. The *only* choices are to secure the door or secure the child. Kids will do their best to meet our expectations. When they don't, that means they can't. She's too little to be responsible for her safety in this way. She's telling you that in the clearest way possible.
It will probably only take a week or two of doing this before she says she's ready to not need the harness anymore... but have the talk about how if she tries to go AWOL again, then you'll keep doing the harness until YOU think she's ready to go without it.