I could not find any relevant information on this, sites just recommended class 10. I want to know if it is okay to use Class 4, 8 GB SD card, for making a bootable Linux drive for Intel Galileo, or even installing Windows. Are there any consequences of doing so?
Any read/write operations will be slower. However, if you don't want to stream Full HD videos or other heavy content that requires that kind of data transmission rates you'll be fine. Most of the time people just tend to recommend the new stuff. Especially in IoT surroundings just using the one that is enough is often better since it's often using less resources.
Depending on the firmware of the Galileo you might even be better off with the older card, since the support for the faster ones was added later and you might have to update it before the c10 even works.
There are 4 standard SDA ratings that I'm aware of: 2, 4, 6, and 10. Basically, the number corresponds to the minimum write speed in MB/s that the card is capable. So for instance, a Class 4 would have a minimum write speed of 4 MB/s, whereas a Class 10 would have a minimum write speed of 10 MB/s.
As Helmar mentioned, this means that the Class 4 will be slower. If you're running well under the 4 MB/s limit, however, as Helmar said, you could be better with the Class 4. That being said, if you're running close to the 4 MB/s limit, one problem I have had when videotaping to SD cards is that the card will start heating up, which can cause damage to the card. Frequently, upgrading the card will resolve that issue.