Where did the free space for my deleted ExFAT partition go?
The command diskutil  resizevolume  /dev/disk2s3  R requires the free space to reside after disk2s3. The command diskutil  erasevolume  "Free Space"  ""  /dev/disk2s2 created free space before disk2s3. There is no or very little free space between disk2s3 and disk2s4. This is why you received the error message.