Caller cannot be heard When the my Nexus 4 is called, the telephone caller can only be heard when loudspeaker is switched on or with earphones - is this normal? - is this a case for warranty send-back?
No, it's not normal. The following might seem obvious if you're familiar with Android, but it's worth a check.

Android has separate volume levels for media (such as music), notifications, and in-call. It also has a separate volume level for when you're using headphones, to avoid deafening yourself by plugging in headphones when the volume is high. It could be that the "in-call without headphones" volume is very low, and the other volumes are normal. Try turning up the volume (with the physical volume keys) while you're in a call (with no headphones plugged in), and see if that solves the problem.

If not, it sounds like a hardware problem with the speaker that goes next to your ear. There is a separate speaker for when it's on speakerphone, or for music or videos. In that case, a warranty claim would be the appropriate course of action, especially if this is a new phone and it has always had this problem.