On behalf of Venumadhav Josyula
If one selects 4G Prefered , then it first attempts to camp of 4G Cell, if it fails then it camps on 3G. The way you do it ( NEXUS 5 :: goto Preferred Network type and select accordingly ), if there is something called PRL list which gives to more options. The way you access prl is dail
*#*#4636#*#* , this might change operator to operator but in case of airtel it works for me.