Why is SQL Server 2012 memory usage limited to 4 GB
Short update: I could solve the problem myself.
Problem was actually not related to the memory. We had high traffic with many very CPU expensive queries which created a constantly high CPU load, which were the reasons for RESOURCE_SEMAPHOREs.

After reducing the quantitiy of queries significantly and tuning the queries itself to be less expensive, the load decreased and there were no more RESOURCE_SEMAPHOREs.

Memory usage is still only 4 GB, but obviously system doesn't need to use more!