How often to flush/change dot 4?

Just don't change to DOT5 It's taking a big chance.

I've worked in Engineering at two different top 5 OEM automotive brake parts suppliers and I am not aware of one of them that will sell a part for a DOT5 silicone fluid system. A lot of the aftermarket companies will also not warranty parts with DOT5. The long term material compatibility is not always there, and also since dot 5 does not absorb water, it can have water pockets form and create corrosion for that reason. The old natural rubber isn't as tough as the EPDM so it's likely you get some swelling that you don't want. You want the water to be in suspension, not in pockets in your system.

If you kept DOT4 and even changed it every 5 years or when you change the pads, whichever comes first, you'll be good. I worked on an 07 Saturn Ion over the weekend that as far as I can tell has never had the original DOT3 flushed after 13 years and 135k miles and there isn't "an issue" either, pistons slide perfectly. Michigan is a terrible place for temperature changes (Yearly swing between -25F and 100F most years, and it's damp) That's no guarantee on the longevity of any of the parts, however, this is still reasonable given the manufacturing of the vehicle. No US car company has any service recommendation for brake fluid flushes in their owners manuals.

DOT5 actually also has some compressibility so it will have at least a small decrease in performance.

If you can pour fluid without spilling and have a tight system DOT5 provides no benefit. If you do spill, just flush the paint with soapy water.

My ATE Typ 200 after 2 years in the car is amber just as brand new. It's not attacking the rubber whatsoever. The darkness is either contamination from dirt or the carbon leaching out of the rubber.