F1801-05201BK-KEMIMOTO Upgrade Motorcycle Saddle Bags 26L

Is the 15.4 inch measurement the width of the bag or a diagonal measurement?

The dimensions of the bag are 33CM x 23CM x 18CM/ 12.3X9.06X7.09inch.

Does this come with the 4 tie down brackets?

No brackets include. The bag will come with 4 nylon cable ties. Please note that some motorcycle models require saddlebag brackets for installation due to limited space between shocks and tail lights.