what should be the minimum write speed of microSD for 30fps 4K, 60 fps 1080p or 100fps 720p etc?
You have to check the specification of your camera.  30Mb/s could be plenty for 60fps 4k or not enough for 24p SD, entirely depending on what format and data rate is being used.  Your camera should say the data rate of whatever you intend to record in.  Use that to determine the needed card speed.  Make sure that the write speed is actually what you need as well as read and write speeds can sometimes differ significantly on SD cards.

According to this page, the Hero4 Silver only supports 15fps 4k recording.  For that particular camera, the required data rate is 45 Mbps.  Note that memory cards are typically advertised for the READ speed which is faster than the write speed in almost all cases.  Verify that the WRITE speed exceeds 45Mbps as the camera needs to be able to catch up on writing if it falls behind.