Default kernel version of Android-x86 ISO I know that android-x86[...]-k49.iso means it is using kernel version 4.9, but for some ISO like android-x86[...]-r4.iso, the kernel version is not mentioned.
Since 64-bit isn't booting for me, it might have kernel version 4.9+ but not 4.19 since 4.19 is also available for download.
How can I know what is the default kernel version?
How can I know what is the default kernel version?

By reading the release notes. With every new release, the development team publishes the major changes compared to the previous releases. The kernel version is mentioned under the Key Features section.
The Release Note for 8.1-r4 indicates:


Update to LTS kernel 4.19.110.


For 8.1-r5:


Update to LTS kernel 4.19.122.


For 9.0-r2:


Update to LTS kernel 4.19.110.


When the iso file is not for the latest version, then the kernel is also mentioned with the filename.
For example, there are 3 64-bit files for 8.1-r4. The first 2 are for the kernel 4.19.110 (not mentioned on the filename), while the third filename has the kernel mentioned - kernel 4.9.


64-bit ISO:  android-x86_64-8.1-r4.iso
64-bit rpm:  android-x86-8.1-r4.x86_64.rpm
64-bit ISO with kernel 4.9:  android-x86_64-8.1-r4-k49.iso