Good Day
I agree with John, good lenses are not cheap even at the 102mm aperture. I have made telescopes with lenses from D&G, Antares and Istar and recommend them. I would expect to pay over $350 for a good 4" lens unless you are lucky enough to find a secondhand bargain.
Istar do 4" f10 and f12 lenses but not f6. As John says you may be better off selling the scope and upgrading to a different model.
It may be worth investigating whether the lens you have cannot be improved with cleaning and adjusting?