I believe Super Saiyan 4 pushes your Saiyan power to its maximum potential. So the only way for Super Saiyan 5 to work is if it takes it even further beyond that.
That doesn't exist outside of fanart, but if considering LBSSJ4 to be the equivalent to Blue Evolution, the multiplier for 5 would then be over 20 times regular SSJ4. But LBSSJ4 pretty much is SSJ5 anyway.