4 lines (1 on each corner) would need to extend out at approximately a 45 degree angle to kind-of hold a boat steady in windy conditions. In a lot of slips, that simply won't be an option. If the bow and stern lines come off at a 90 degree angle, there will be nothing to stop a boat from moving fore and aft until the boat moves far enough to change the angle significantly from 90 degrees.
The primary purpose of spring lines is to limit fore and aft movement of the boat.
If you use short breast lines on the bow and stern, they can work
in modest conditions but are not well suited to tides or if there is storm. Because they are at a poor angle, the forces on the lines and cleats
tend to be much higher. Even more important is they are short, they don't provide much shock absorbing effect. In calm conditions, you will feel the boat jerk now and then but in storm conditions, that jerk can easily pull a cleat out of the deck
Ideal layout is to have 6 lines:
- 2 Bow lines, crossed (but not chafing on anything), to provide a good length to all for stretch. This keeps the bow from moving side to side.
- 2 stern lines...same thing but to keep the stern from moving side to side.
- 2 spring lines to keep the boat from moving forward or aft.
Obviously, you need to allow a little movement. This setup gives you a lot of flexibility and because the distance is significant, it can absorb a significant tide without the lines going slack or bar tight.
If you are at a floating dock, you can cheat a little as you don't need to account for tides. Likewise if the tidal range is small...but even there, it's not a bad thing as the unexpected can happen. On Lake Erie, we were in a marina one night with a bad storm. By morning the boat was 7-9ft down sitting on the bottom as the water
was literally blown out of the marina. We had to adjust lines 4 separate times. Lots of boats with cleats
pulled out as the staff couldn't keep up with adjusting the lines as most owners were gone.