I would agree that 4" is too narrow but it might depend on other factors....can you post a pic of the ramp or where it will go?
I made mine about 10" wide with cleats 4-6" apart and tried to keep the angle as low as possible. The steeper it is, the closer together the cleats should be.
I highly recommend using a hinge to attach ramp so you can adjust angle as needed, I used a door hinge on the one
You can use a block under the end, they can easily and safely jump down 16", and they usually jump up about halfway when going back up.
These ramps are a little too steep and cleats a little to far apart and they are not hinged,
as I needed to put them up like right now and they've worked so I left them.
The platform is about 16" high.
Their feet slide a bit, but haven't seen any foot problems because of it.
Even babies need ramps sometimes.
