Settlers of Catan: Cities and Knights - Rule Clarificarions
Correct, you cannot upgrade to another level 4 unless you first build another city. From the FAQ:


  City Improvements and Metropolises - May I upgrade a city to 2 metropolises at the same time?
  
  
    No. If you only have 1 city and already upgraded it to a metropolis, you are not allowed to build a 4th level of improvement in another area. Before you do this, you must upgrade another settlement to a city.
  


Although the question being asked is about placing 2 metropolises on one city, the answer provided specifically states that you are not allowed to build a 4th level of improvement in another area until you upgrade another settlement into a city. 

This same question generated some discussion on BGG a while ago; most people seemed to agree with this interpretation, even though it is not fully clear.