Is it possible to use exploratory factor analysis to extract four factors, and then combine these four factors (i.e. the factor scores for each observation) into a composite scale?
Yes, you can. Exploratory factor analysis aims to reducing data to a smaller set of summary variables... which are not the initial factors. But, depending on the EFA you used, you can use, for instance, a linear model based on the EFA score to reduce some factors in one. But, you should discrib a little bit more what analysis you think to use.