I plan to bring in 4 Kg silver items (Appx 4500$) into USA. have receipt for all the items. Is there a cap on how much I can bring in? I read the duty is 3% on the amount over $800. Is this correct? I guess I need to fill in the declaration form and handover to the CBP agent? Anything else I should be doing? Thanks

That is the general idea but there are so many factors to take into consideration you should not rely on that in planning for how much you might possibly potentially owe. You do not specify what the items are, where they are from, whether a joint declaration will be used, nor a host of other factors (such as if they are subject to 301 authority, or fall under GSP, etc.) all of which are highly important in determining a) if the items are even allowed to be brought into the country and b) what the amount of duty might be. Also, there may be other taxes due in addition to any duty owed.

Fortunately, Customs has this provided this webpage Customs Duty Information | U.S. Customs and Border Protection (cbp.gov) to help you. And, in the off-chance the items are considered bullion or monetary instruments CPB offers this helpful webpage: Regulations for importing bullion, gold coins, and medals into the United States (cbp.gov)

It can be very complicated and I would contact Customs directly and get accurate information directly from the horses mouth so you can plan accordingly. Strongly suggest not fooling around with making a less than accurate/truthful declaration - that frequently results in the goods being seized and ultimately forfeited to the gubmint.

You could also contact a customs broker to help you determine what the costs will be but you will probably pay a fee for that service.