A Rocna 20 is probably capable of holding with greater force than 4000 lbs., since the Rocna 15 was generating loads of over 5000 lbs. in the Sail magazine anchor test a couple years ago. However, a permanent mooring is a far better idea for a long-term solution.
How well a round shaft helix will work depends a lot on what the bottom composition is like and how deeply you can screw the helix anchor into the bottom. Without knowing what the bottom consists of, you're basically shooting in the dark. If it can hold 4000 lbs. of force, then it is probably sufficient for your boat, but you'd also have to size the chain and pendant properly.
See this page from the Hamilton Marine catalog regarding that:
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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