Dental implants should be placed 1.5-2 mm away from a natural tooth to prevent damage to the tooth/ it's ligament. You should see your dentist to evaluate the tooth and rule out infection around the implant.
Yes, it is possible to place 4 individual implants in the front to replace 4 teeth. But it is usually not necessary to do that. Doing two implants and a bridge works very well for most patients.