Silent while gaming, or silent while desktop/media?
Because the latter is easy: any decent card with 0dB (fan stop, fan idle, idle stop, etc). Not all will be perfect (some have poor hysteresis), but starting from there will get you to the card you want.
The former... I'd venture it's not going to be pretty. The RTX cards are probably the best when it comes to power efficiency for the performance they have, but all of them still draw a large amount of power. Any cooling solution for one of them under load will be a bit noisy in the Dan A4 SFX, solely due to the limited "width" for GPUs (it is a very focused case, after all). 1 slot coolers are generally noisy and 2 slot coolers place the fan very close to the side panel.
That being said, it's not LOUD loud, but your tolerance for it will completely decide the outcome of this.
I'd guess something wild like a mITX card with a 120mm AIO might work, but I have no clue what I'm going on about at that point.
