Do I need to declare or pay US duty on personal, used computers?

It depends whether you intend to take this equipment back when leaving the US, or not.

If you intend to take the equipment back, it is not dutiable. There is no need to declare it unless it falls into other declaration categories (i.e. food), or unless the CBP officer asks you about it. Note that custom declaration only asks visitors about the goods which will remain in US.

However if you intend to leave some of the equipment in the US, you need to declare this and its value. Then you'd have to pay the assessed duty.

PS. Your situation is not unique at all; any tourist which flies in with a latest iPhone, MacBook and iPad brings in around $4k worth of goods. And there are plenty of those.

PPS. Generally if you get worried whether something should be declared or not, just tell the CBP officer you have them. Not only you're following the law (as you declared goods to the officer, you can't be convicted of contraband even if the good itself is prohibited - i.e. meat), but from my experience CBP gives you more leeway in cases where it is questionable whether duty should be assessed - i.e. if you bring a lot of alcohol.

Since these items are for your personal/professional use and you are going to bring these items back to your home country, these would be considered Household effects, thus are duty free - provided you aren't bringing them to sale, or gift to someone. Duties are usually waived for items over a year old.

You will need to declare the items and value on the Customs Declaration Form 6059B, most likely on the airplane, or before going through Customs.

Side note: From my experience, many countries are much stricter than the USA about electronics, but USA is way more lax (as long as they aren't prohibited) because they know you can usually buy this stuff cheaper in the US anyways.

If you aren't bringing electronics made in an embargoed country or on the prohibited list, you are OK.