There are 2 partition styles most commonly used to partition disk, MBR and GPT. They are different slightly in supported partition amounts, disk capacity, OS compatibility and boot mode. For example, GPT can support disk larger than 2TB but MBR partition style cannot. And this is why your 4TB hard drive only shows 2TB.
So, to restore all the capacity of the hard drive, you need to convert the MBR disk to GPT disk. In the following content, let me walk you through how to perform such a conversion (Windows 11 only supports UEFI, so you need to convert MBR to GPT).
1. Convert 4TB drive to GPT with Diskpart (data loss)
There is a built-in tool called Diskpart, which can convert 4TB hard drive to GPT with several lines of command. Here are the detailed steps.
✍Note: this method needs to clean your disk before converting, so if it is not a new hard drive or has important data on it, you have to back up the disk beforehand.
Step 1. Press “WIN+R”, type “CMD” and press Enter to open the command prompt. Then, input
"diskpart" and hit Enter.
Step 2. Type the following commands, remember press Enter after you input each one.
select disk n (n means the drive letter, replace it with your 4TB drive letter)
clean (clear all partitions on the selected disk)
When the “DiskPart successfully converted the selected disk to GPT format” shows on the screen, type “exit” and press “Enter” to leave the program, and check if the rest of the capacity is restored.