I am building a 32 X 48 post frame home with 8 feet between post. Will I have trouble if I bolt wood I beams to the post and build the floor on top of them? I want the floor 4 feet off the ground for easy access to plumming, wires etc. If this is OK will I need to have post in the middle of each joist for added support?

You really need to consult an architect or structural engineer for this. Do you have a permit and who will be inspecting this home, the county or city?

I'd consider changing the width to something like 31'-6".
This would allow for two 16' joist to cover span with enough overlap at center.

If you go the full 32' width, it would take a 16' and a 18' joist to cover span.

... and check with local codes.

See attached PDF file.