The problem with other answers here so far is that the OP explicitly states that he is aware that tracking can be done on 2G technology, but is wondering about 3G and 4G technology.
GSM and LTE encryption works using symmetric or asymmetric encryption methodologies. This means that there are three ways to obtain the key needed for snooping
- Obtain the public key or shared secret from the phone
- Obtain the private key or shared secret from the phone company
- Brute force encryption algorithms
While GSM encryption may have been cracked, more modern ciphers are now in use so method 2 has been re-secured. Using a rolling key (Perfect Forward Secrecy) could also help to secure this communication channel such that cracking the protocol in hours could still be useless if you have moved on to a new key. This means method 2) is fairly secure until some algorithmic weakness is discovered or brute forcing capabilities provide for faster cracking.
As for method 1) in order to attack via this means, one would need to hack the UICC (Most people call it a SIM card, but this is not technically correct - a UICC can house multiple SIMs). These are stored with some pretty strong encryption making cracking impractical with current knowledge and technology and if their is suspicion of cracking, the cell companies can re-key the SIM rendering that work moot.
The final method is obtaining the keys from the cell phone company and using that private key to snoop a conversation. This can be done on 4G technology using a device known as a "Hailstorm". Cities like Oakland and others are having to upgrade their old stingray units to Hailstorm units in order to cope with the end of the 2G networks.
Ars Technica has a pretty good overview and survey of the history of Stingray/Hailstorm and related technology. Thus, the long-story-short here is that the Hailstorm has the private keys used by major cell phone companies and can intercept traffic using method 3) using this device for even LTE/4G traffic.