Main factors are handlebar widths, and whether you can alternate the height of the hooks. If you have hooks at the same height, chances are one bikes handlebars will touch the next bike's saddle. If you can have ~30cm vertical difference between the hooks, you should be able to squeeze four in. Three if not. There's a picture of my rack on page 34, 3 bikes in 103cm.
Main factors are handlebar widths, and whether you can alternate the height of the hooks. If you have hooks at the same height, chances are one bikes handlebars will touch the next bike's saddle. If you can have ~30cm vertical difference between the hooks, you should be able to squeeze four in. Three if not. There's a picture of my rack on page 34, 3 bikes in 103cm.