Will a base tan help me prevent a sunburn?

A base tan might give you an SPF of 4. So if it took 20 minutes for your untanned skin to burn, then you might burn after 80 minutes with a base tan. This doesn't mean you will automatically "prevent" a sunburn, though. You can get a sunburn even if your skin is darker. Also, tanning is not a safe method of prevention. Tanning causes skin damage and aging, and can be linked to skin cancer. So, it is far better to use a sunscreen to avoid a sunburn than a base tan.