Last week I am installing a 4 foot antenna on my boat that has a bimini top and I’m concerned about how the reception will be. My mom suggested me a lot new things to do before getting into it. She suggested me that the higher you can get that antenna the better your reception will be. She said to think creatively, and find a spot on the bimini if it is up most of the time or outside the bim if you bring it up and down a lot. As a rule, you should always mount an antenna away from other metal, the longer the antenna, the better the performance.
The Best option, use the 8' antenna and place higher than your bimini top. Next best would be place the 4' outside and above your top. There is a certain parasitic action which takes place between antennas and surrounding metal, it is best to avoid it. The actual loss and deformation of the antenna pattern would be difficult to explain but the radiation pattern from an antenna is like a donut around the antenna, you put another piece of metal in the beam pattern and it is like taking a bite out of the donut, this will be an area of poor reception and transmission coverage.