I have bought a raspberry pi 4 B. But I don't have a powerfull enough power suply, and won't be able to get one in a couple of days. IS
It’s not a good thing to do.
At any point the WiFi network could drop, the processor reset, the SD card get corrupt. It took me nearly 3 weeks to identify what was causing me issues with my 3B+ before I twigged on and bought a power supply and not a charger.
Save yourself a lot of frustration and get the correct supply first.
The worst that can happen is that the Pi would crash.
The Pi itself will run even with the warning (as it only needs 3.3V) USB peripherals may not work.
I have tested various Pi models with variable voltages. All work down to 4V See Raspberry Pi Power Limitations
NOTE I do not recommend this as normal practice.
The risk of OS corruption due to low power is greatly exaggerated. Only removing power while the Pi is actively writing to SD Card poses a risk, but even then the journalling generally recovers the file system.