Project duration two hour maximum?

Besides all the above, I have to say that editing four hours, or even two, in a single Motion project seems to be a very hard job. If this is generating Motion graphics, I'd suggest doing it piecemeal. Such a large project is likely to have high memory and GPU use and may bog you down.

And if this is mostly editing video, it is probably much more easily done in FCP.

If you give us some notion of what you are trying to achieve, maybe we can offer better advice.

With a 60fps project, it appears that 2 hours is the limit.

I had no problem changing to 4 hours for a 25fps project, so I guess there may be a limit not on duration but on the total number of frames.

There is a limit of 432,000 frames regardless of framerate. That's 2 hours of 60, 4 hours of 30, 4 hours 48 minutes of 25, 5 hours of 24, 8 hours of 15 and 12 hours of 10.

The calculation for the others is: 432000 ÷ framerate ÷ 60 ÷ 60.

If you're projects are dependent on time and frames, do not ever use fractional frame rates (it's an artificial way of counting frames). Whatever your project frame rate is, it will be conformed in Final Cut.

Replying to myself: yes, it appears that my guess was right. At 25fps the limit is at 4 hours and 48 minutes, which is exactly the same number of frames as 2 hours at 60fps.

I see, that is something you may want to do in Motion, but I don’t see any reason to use 60fps. If you choose 30 or 25fps it should be more than enough and will let you reach 4 hours.

Audio file does not have fps.

If this was an mp3 or aac file maybe you meant 144Kbps?