1. pi3 will probably be running stretch pi4 will only boot with buster , so not you can't use 1 SD card for both.
2. Safest option load second SD card with buster and boot pi4 install all required software , copy any programs you have written and any data you want to keep from pi3 to pi4 over network or by mounting pi3 SD card in reader and attaching to pi4.
Upgrading SD card from pi3 stretch to buster is not recommended as it can and often does lead to problems.
We want information… information… information........................no information no help
The use of crystal balls & mind reading are not supported