Can I use N1ED without Bootstrap?

Yes, you can!

Way 1. Turn off Bootstrap 4. You can just not include optional plugin Bootstrap Editor which comes in a bundle with N1ED.

Way 2. Use Bootstrap 4 for content only. Include Bootstrap 4 to your current theme and use Bootstrap Editor for creating content as usual. In most of the cases your non-Bootstrap theme will support fine internal content styled with Bootstrap 4. In other cases you will need to adapt your theme a little.