I am interested in getting the UPS PIco HV4.0B/C Stack for use
only with the super caps, as a stay in vehicle raspberry

There are just a couple of options for super caps when ordering
the HV4 right now, the biggest being the 3000F.  I see that hats
were once an option.

The 3000F is rated @2V7, and need to be
connected to the Super Capacitor pads. This Super Capacitor can
be used with battery or without (alone).

We have also an additional Super Capacitor Ratted 4000F
but @4V2. Currently out of stock.
This need to be connected to the Battery Socket, and CANNOT
be  used with the battery. Only alone.

I have a couple of questions.

How long can the 3000f cap provide power to the rpi4?

The 3000F is comparable to  1000 - 1200 mAh
LiPO Battery, and depending to running application is about
30-50 minutes of work.
The 4000F is comparable to  2000 - 2200 mAh
LiPO Battery, and depending to running application is about 50
- 80 minutes of work.

It seems the customer needs to assemble/solder the caps on.  Is
there instructions and hopefully a picture I could look at before
ordering to see if I have the ability to put it together?

Yes, in the manual, please download it and check,
due to size of them I would suggest to use cables. If you buy it
form us, we will add a socket, plug and cable to them.

With the cap hats, is everything all together, just stack on top
of the gpio pins?

Yes, but you need to  use cable fro SC connection
, so much more flexible

If so, do you know when the cap hats will be available again?

The 3000F are on stock about 30 pcs, the 4000F we
will have again next year, second  half of January 2025