How many wires can safely be secured in a Marrette 33 wire nut?
Here is a catalog from Thomas &amp; Betts, the makers of Marrette connectors. Pages 14-15 cover the black high-temperature wire nuts including the Model 33. 

The graph on Page 15 shows limits of "Min. 1 #18 + 1 #14 Max. 4 #14", so four 14AWG wires is exactly at the manufacturer's limit. 



There's also a detailed table at the end of the document (Model 33 is covered on pages 25-26) which tops out at 2 #14 + 1 #12, 4 #14, or 2 #12.