Definitely suitable for any surface a reasonable pilot would call a runway, but I wouldn't personally land in an unchecked area because the gear isn't designed to take abuse from rocks, holes, etc like you see in bush flying videos.
I do positive G acro in my -4 with passengers all the time, but you do have to observe the lower gross weight limits (or adjust G limits) for acro, and CG can become an issue fairly quickly in a -4. I never do negative stuff, and never do snaps. Even a light -4 will weigh around 1100 lbs with full fuel, and Van's recommended gross weight for 6 G's is 1300 lbs. You can lose fuel, lose cockpit weight, lose G margin (<3 G loops can be done in a -4), or pretend Van was too conservative with his limits. (Please don't do the last without *informed* consent from your passenger.)