You definitely want two sticks for optimal memory bandwidth (the exception to this rule would be AMD Brazos-based laptops, which only have a single-channel memory controller). Sometimes the price for RAM from an OEM like Lenovo is so ridiculous that if you want, say, 8GB it's actually cheaper to specify the lowest possible RAM (one 2GB stick in this case) and then remove it (possibly re-selling it) and replace it with two larger sticks (two 4GB SODIMMs) rather than spec'ing one 4GB stick and then buying a matching one in the aftermarket. Looking at Lenovo's site, it's not quite that bad at the moment, though it's close: the upgrade from one 2GB stick to one 4GB stick is $80, whereas two compatible 4GB sticks is
$93 from Crucial -- and you can certainly find close to $80
on NewEgg and probably could get below that with coupons or a sale.