The rules of Cities and Knights state the following:
"...you may not purchase any improvements beyond the third level of a given color unless you have a city where you could build a metropolis."
Let's say someone has a metropolis built for Science (level 4) and I am currently holding a metropolis for Trade (level 4) though own no other cities. Does the rule imply that, even though the metropolis has already been taken for Science, I still cannot go past level 3 for Science?