Hello.
I can make this. I have made several (indoor) models of autonomous robots in the past, with indoor GPS-like navigation.
But in any case you will need to use sensors because at the long distance (and especially on the grass, with 4 non-turnable wheels) you will have an accumulated error. That means you need to refresh the position and direction of your cart.
At least, you will need an accelerometer and an odometer, but I would propose to use cheap GPS or, at least, a compass.
Also, I would propose to have a simplest remote control (believe me, that's a very useful thing, especially on the debug stage :-), saves kilometers of jogging when the cart goes somewhere you didn't expect.
And that's convenient to have some wireless info at debug stage, even when it will be well-tested at my side.