Where the 4th amendment applies here is if any evidence of a different crime were found while looking for information on the first. If the phone records were acquired by a warrant, that evidence would not be admissible in court.
A warrant involves judicial oversight. Warrantless searches do not.
Step outside and talk to them if they want to interview you, and password protect your phone and demand they get a warrant if they want to look at it.
That’s why there is the Miranda warning, for example.
You should know and protect your rights. Yes. AND the police should not as a matter of course attempt to coerce you into giving them up.
One is being the accuser, to whom evidence is asked about the validity of the accusation (this law proposal). The other is to automatically accuse someone (in your proposal) and demand evidence to deem them innocent.
But as I noted in another comment, a warrant involves judicial oversight.
The police will already have access to the suspect's phone.
Now obviously, the easiest and cheapest thing for the taxpayer would be to simply lock up the partner when a woman with a partner is murdered.
That would be unjust, even if it is nearly always right and easiest and cheapest. Ok, but MRA folks would go ballistic, so let’s make it easier:
If a woman with a male partner is murdered, we simply require the male partner to give up their cell phone’s contents. No silly paperwork for a warrant, no laws about self-incrimination or any of that, we just cite the “Taxpayer’s Efficiency Act of 2019” and they have to hand it over, or sit in jail for contempt until they do.
Is that still unjust?
It is, isn’t it?
I put it to you that if false accusations of rape, which happen rarely, justify examining phones belonging to the victims, then murder of a woman by her male partner, who is usually the perpetrator, justifies the exact same thing.
What stops us in both cases is the notion of justice. It is unjust whether the person is likely or unlikely to b a law-breaker. And we as a society have decided that justice is a greater good than saving a few bucks or making things easier for police.