Ram in a system are like shoes on your feet. When you loose one shoe the shop from which you bought it from won't sell you just one but a pair. If you own one 4GB stick and you'd like to add more then its best you buy an exactly similar 4GB stick with the same specs so it gives you optimal performance from your ultrabook. As per the maximum 8GB capacity, that limit has been set by Lenovo's (mother)board maker and if the
product spec sheet states its maximum limit is 8GB then we have to go by what's been stated. Ofc there is a way you can find out if does support 8GB per slot and that is by borrowing a 16GB kit from a friend and testing it out and seeing if your ultrabook boots past BIOS screen.
If you're going for a ram upgrade then this should be noted,
DDR3L-1333 MHz/1600 MHz (max 8 GB)
^ whereby the
L after
DDR3 signifies that they are low voltage kits. Kits of that range come rated at 1.35v for operational voltage. Issues with adding just a 8GB stick to your unit can cause it to become sluggish and also they won't be balanced as the shoe.