Four monitor desktop (or laptop)

four 2560 x 1440 monitors? I don't think so.

a GTX 1080ti card will support three monitors at that resolution.

I don't know of another card that supports more than this.

There are video cards (with ridiculous prices!)  that support 4 monitors but you'd have to purchase it separately from the PC. So make sure the card will fit inside the PC and that the PC's power supply is big enough.

Some Dell PCs will allow you to connect one monitor to an onboard video port while the others are connected to an add-in video card. So maybe you only need an add-in card that supports 3 monitors. You'd have to investigate what resolution an onboard (eg, HDMI) port will support with 3 monitors on the add-in card.

Alternatively, you can probably use a USB>Video dongle for the 4th monitor, where "Video" is one of the ports supported by your monitor, instead of using an onboard video port. But again, you'd have to see what resolution the USB>Video dongle will support.

If you go for a USB dongle, get one that's USB3 because that will give you better performance than a USB2 dongle, even if it costs somewhat more, since decent PCs all have USB3 ports these days.

Drive up to four ultra high resolution displays with the Quadro NVS 450 (This is NOT a gaming Card)

http://www.nvidia.com/object/product_quadro_nvs_450_us.html

Max Monitors Supported 4
4 x DisplayPort
4 x DVI-D (dual link) - with adapter
Max Resolution Details
DVI: 1920 x 1200 / DisplayPort: 2560 x 1600

https://www.amazon.com/Smart-Nvidia-Quadro-450-Adapts/dp/B0096FJ0EC/

This fits in MINI TOWER versions of Dell Optiplex, Inspiron,  Dimension, etc.

The card uses 35W and would work in an INSPIRON 3650 for example.

The modern version of this card is the NVS 510

http://www.nvidia.com/object/nvs-510-graphics-card.html

It uses 35W so it too will fit into most mini towers.

2.70” H x 6.30” L, Single Slot

Again this is NOT a gamer card.

https://www.amazon.com/NVIDIA-NVS-Graphics-Card-0B47077/dp/B00BNHKO50