How would I replace 4 posts and beam holding my covered patio with 2 posts?
The vertical posts are probably almost strong enough for only two supports as is.  Replacing those with steel is not necessarily needed.

The challenge is the strength of the horizontal span with reduced supports.  If left as wood, it will have to be fortified by a factor of more than two.  Of course, the added weight of that much more beam means the factor must be increased a little more.

Assuming #1 northern Doug-fir for a span holding roofing only which is 20 lbs/sq ft live load plus 10 lbs/sq ft dead load and assuming that the current spans were engineered at 150%, then increasing the current 10-11 ft spans to 30 feet would require replacing the pair of 2x6s with a pair of 2x12s on edge, firmly tied together.

I tried to figure out an equivalent steel beam, but I am out of my experience.  The first result says that a 4x4.5 inch beam is more than twice what is needed.  Searching for rough pricing for such beams is surprisingly challenging.