A newly mounted or completely deflated 4.80/4.00 high speed tire such as found on a boat trailer can have as much as 2" of space between tire bead and rim..... How does one seat the bead all the way around the rim on both sides and hold in position prior to inflation?
A shop uses a large surge of air into this area to help seat the bead. Many monster truck teams have been known to actually use butane to make a small explosion to set their beads ( I am NOT recommending this).
At home methods. First take the rim of the trailer or vehicle. If you don't have a actual air supply (ie compressor) you may consider just bring it into a tire shop. How ever many times you can take a fairly think rope around the center of the tread and using a board or poll start twisting to tighten the rope and pulling the tread inward. This many times will deflect the beads outward to the area of the rim were the air can seal enough to start seating the bead. Once this occurs you can just fill the tire.