A curbless without a door requires a larger "wet area" since water can splash out 4-5 feet.
I just did our 4'x4' shower. I planned a curbless and even recessed the concrete 1'-1/2" in that area to allow enough room to build up. Shwoer drain in the center. At the last minute I decided against it. 3 reasons:
1. I wasn't confident 4'x4' was a large enough wet area to ensure water wouldn't escape.
2. I realized we wouldn't be able to have a bath mat outside the shower door because the glass door bottom gasket would have to touch the tile and wouldn't clear the mat.
3. The inspector couldn't give me a straight answer on how to meet code with a curbless. Him... "you need a 4 inch curb" Me..."I want an ADA accessible shower. Him..."hmmm, I'm not sure I'll have the other inspector get back to you." I know people do it all the time. I just didn't have the time to go through all the back and forth.
So, I decided to drop a low profile Kohler 4'x4' shower pan in the recessed area and ended up with about 1/2" curb. I never asked if it would meet code or not (maybe a bit risky). I had 100% confidence it would be waterproof (I was only about 95% confident in my tile work that it would be). It gave me enough curb for my shower door to clear a shower mat and although I haven't used the shower yet, I'm confident water won't be escaping out into the rest of the bathroom.