I have a 600 F4 as well as the 500PF. The PF is lighter and easily hand held. Not so for the 600 F4. Aside from the extra 100mm the 600F4 works well with a TC. not so the 500 PF The TC really slows down the AF on the 500 PF.
As far as the trade goes, I don't know the condition of 500 F4 G and if it is Nikon US or gray market. Assuming very good condition and Nikon US, then based upon the prices I found on Fred Miranda Buy and Sell, I would say your lens is worth more than $4000, perhaps as much as $4200-4400. To sell it you would need to absorb paypal fees and shipping but even with these you would be gross over paying for hte 500 PF
As far as the trade goes, I don't know the condition of 500 F4 G and if it is Nikon US or gray market. Assuming very good condition and Nikon US, then based upon the prices I found on Fred Miranda Buy and Sell, I would say your lens is worth more than $4000, perhaps as much as $4200-4400. To sell it you would need to absorb paypal fees and shipping but even with these you would be gross over paying for hte 500 PF