You have a petcock on the radiator.
In theory you have 4L of new coolant and 2.5L of water/residual coolant. Roughly 60:40 for maths.
If you drained 1.25L of coolant from the radiator. You’d remove .75L of new coolant. If you add distilled water, you’ll be closer to 50:50.
A smarter way is to get an inexpensive refractometer, and test the coolant. Maybe wait a week to let the coolant in the overflow reservoir mix.
If we segue for a moment, look up toothpaste abrasivity or toothpaste RDA, and tooth and gum damage. You’ll get bombarded with a litany of fear mongering BS about how whitening toothpaste is going to wear your teeth and gums away. It’s nonsense, it’s just fear mongering, because there is no evidence provided. There’s evidence that a tiny portion of the population needs to be cognizant of toothpaste abrasivity, that doesn’t justify applying it to the entirety of the group, or not acknowledging that the ADA and FDA recommendations are safe based off of scientific data and a lack of real world evidence to the contrary.
If a high RDA toothpaste sensitizes your teeth, you don’t need a scientific study to know it’s bad for you as an individual. Likewise if you’re sensitive to VOCs, you don’t need a scientific study to tell you off gassing is bad for you.
There’s a litany of research about cars off gassing. There are no deleterious health affects linked to the general population. No deaths, no injuries, no cancer, no illnesses.
https://www.sciencedirect.com/science/article/pii/S016041202100026X
What’s indisputable not only in the first world, but the entire world is that motor vehicles are a leading cause of death and injury. If you have access to clean drinking water, do not live in a war zone, and don’t abuse alcohol or drugs, one of the most likely reasons you won’t make it to 60 years old is a motor vehicle transferring an lethal dose of kinetic energy into your body. Acute exposure to non-lethal doses of off gassing products isn’t on the list.
Any toxic chemical exposure is bad, the safe limit is always zero. But whatever that risk is, it needs to be juxtaposed against reality. New car chemicals versus a vehicle that meets and exceeds the latest safety standards and is in a condition to most likely be free of defect, corrosion, damage or repair, that could impede those safety systems.
Your car’s oil pump puts out something like 6-8gpm at idle. If a filter wasn’t installed or was double gasketed, it would take about 15 seconds to pump all the oil out of the sump.
If you filled it back up with oil and there’s no filter, there’s not going to be any mystery about what’s going on.
If someone installed a 20mm threaded filter on your 3/4” Toyota, you could eject a filter while driving. You would be out of oil in a matter of seconds.
Could the oil change place screw up? Yes. Could your filter have leaked oil? Yes. Did you drive 15 miles without a filter? No. Did you add oil, start the car, and not notice the filter was missing? No.
You’re going to have to fight a real uphill battle to show that you didn’t your notoriously junk Toyota 2AZ out of oil, and are trying to run a scam on the quick lube for a new engine.