I tried to buy a Dell Thunderbolt 4 card from Dell but was told it was not available. They offered Thunderbolt 3 but I doubt that it was tested for compatibility. Previous generations of the 3600 series had a Thunderbolt option entry in the BIOS and a Thunderbolt driver but 3660 does not have that. It appears that Dell overpromised and underdelivered in this case.

The Precision 3660 does in fact support a Thunderbolt 4 card. The card can be purchased separately or as part of a kit - GET THE KIT. The kit contains the card, a cable (required) connected between the card and the Thunderbolt header on the 3660 motherboard, and two displayport short cables that go between the Thunderbolt external Displayport connectors and the Displayport connectors on your graphics card if you want to use a dock instead of the graphics card's ports. The Dell part number to order for the kit is 540-BDLC. They are currently in stock. The kit costs under $100 from Dell. Installation is pretty self explanatory, since the cable between the card and the motherboard comes with it's ends labeled. The bill of materials for the 540-BDLC kit is:

     Qty     P/N         DESC

     =====================================================

       1      7R4VG     CRD, NETWK, I/O, PCIE, TBT4, FH, 20H

       1     2WY06     ASSY, CBL, GPIO + USB2, TBT4, MTX

       2     C19Y0      ASSY, CBL, DPI, 4A, TBT4/USB4

     =====================================================

As an aside, you can use the StarTech TBT3TBTADAP adapter to go from the Dell card (which is backwards compatible with Thunderbolt 3) to a Thunderbolt 2 or 1 peripheral. I use the Dell 540-BDLC Thunderbolt 4 kit to talk between my Precision 3660 and a National Instruments PXIe crate (NI SD-200/PXIe-1071) containing a PXIe-8379 card ( which is a Thunderbolt 2 MXI controller for PXIe) using the TBT3TBTADAP adapter and it works.

There is considerable discussion on the net that the TPTDTBTADAP is not bidirectional - the web page *does* say it's not bidirectional - but that is NOT TRUE. I asked StarTech wghat that statement on their web page meant - they said what they mean by 'bidirectional'  is that it will adapt a Thunderbolt 3 compatible host to Thunderbolt 1 and Thunderbolt 2 peripherals; but will *not* adapt a Thunderbolt 1 or 2 host to support Thunderbolt 3 or 4 peripherals. An extremely poor choice of words ('bidirectional') but probably a translation error.

One other thing - if you have a graphics card it may interfere with placing the TB4 card in the right-most slot (looking from the rear) and that may make it impossible to use the 2WY06 cable, because it's designed only to reach the motherboard from that slot. You'll have to find a graphics card that allows that PCIe slot to be used. My GTX 3080 card was too wide... sigh... had to pull it!

Is there any Thunderbolt 4 card available for Precision 3660 at all? Does it require cabling or BIOS modification?

The motherboard does have a 10-pin thunderbolt header (photo below).

Dell doesn't offer any manuals or documentation on ANY of their thunderbolt add-in cards online and support cannot seem to verify if the cards they carry will work.

Still no update from Dell support - despite sending a DM to their support team.

Page 67 of the service manual (https://dl.dell.com/content/manual59803069-precision-3660-tower-service-manual.pdf?language=en-us&ps=true) shows a diagram of the motherboard... and then gives no key to reference the numbered components on the board on the page prior, after, or the same page.

Item #22 in the below screenshot is labeled TBT on the board itself:

Life on the bleeding edge...

Sorry, I don't have any add'l info related, but share your concern.

I did find This webpage from www.mattmillman.com  to be helpful in understanding the complexity in general of adding thunderbolt when it's not part of the system you take out-of-the-box.

Thank you for posting & I will be watching for updates/solutions. 

Also - I don't think any Thunderbolt 3 or 4 will work until/if Dell updates BIOS to provide support.

See https://www.dell.com/community/Precision-Fixed-Workstations/Thunderbolt-3-PCIe-card-for-Precision-3640-tower/td-p/7753427

Unless it's hidden somewhere, I don't see support in the latest BIOS release for 3660 for any type of Thunderbolt. 

The 3660 BIOS is more than BIOS, kind of integrated miniOS/GUI shell for all things pre-boot. Nearly impossible to use without a mouse (and my everyday mouse, a wired USB Kensington Pro-fit full -size is impossible to use with the BIOS GUI - tiny movement of mouse moves pointer across the screen - I have to use a temporary mouse ....)

Also a very strange and misleading marketing information from Dell Corp.: They mention a thunderbolt card to order with the Precision 3460 configuration but will probably send a D-SUB 9 pin card (link)

Precsision 3460ff:

Precision 3660: