I have had an initial consultation for two bridges on implants and the dentist suggests a four unit bridge on two implants in each case. is this ok?

Most likely OK: There is a principle in restorative/prosthetic dentistry in regards to length of bridgework. It deals with what teeth are missing and the root surface area. In your case, since the bridges are implant supported, this principle is not as critical, but the basics of mechanics of tooth chewing forces still need to be addressed. As long as the span between implants isn't crazy big, you will be ok

Probably: Location, bone support, length of edentulous area, angulation of implants, occlusal load, opposing dentition, parafunctional habits, are but a few of the considerations in planing a prosthesis. Without full examination, including models and x-rays, it's impossible to answer your question over the computer. If you have concerns see your Dentist or seek second opinion.