Grab more RAM. 4GB is EXTREMELY low. Get 8GB at the very least, if not, 16GB. Keep it at DDR4.
Consider using a modern i7 CPU or an AMD Ryzen CPU. Streaming requires a lot of work on the CPU, so it's recommended that you get a powerful CPU.
I am not sure if graphics cards are required in a streaming PC, but I believe they relieve video-related work from the CPU.
Also note: To stream at 720p 60fps, you need to have a fast enough upload speed. Above 6mbps upload speed should be good enough for 720p 60fps.
Hope this helped!
-Lachlan