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#21
You only see 4 partitions in Disk Management because what is probably your 3rd partition, the Microsoft System Reserved (MSR) partition (should be 128 MB if it originated in Windows 8/8.1) will not show in Disk Management. That's how I knew your last partition shown in DM would be numbered 5.
If you execute the following lines from an elevated command prompt:
diskpart
select disk 2
list par
You will see 5 partitions listed, the 3rd (I predict) being the MSR.
The very first partition (old WinRE from prior operating system) is the most expendable partition there. The only way deleting that one would be of consequence is if you still have the option (within 30 days of upgrade) to roll back to the old OS, then that partition wouldn't be there to provide advanced boot and recovery options under the old OS.
5th partition can be deleted but understand that this will do away with the present Windows Recovery Environment (advanced boot and all those options) capabilities.
2nd partition has to stay.
3rd partition is what Microsoft apparently intends for highly specialized and future usage and you'll will find its necessity is often debated, sometimes hotly, here at TenForums. Mine from Win10 clean install is only 16 MB so I just let it be and don't get into the debate.
4th stays of course.
Hope that helps you decide what you want to do.
BTW: Regarding Macrium Reflect - you may want to try checking the options (under Other Tasks/Edit Defaults) to not use VSS, this sometimes provides a workaround against the kind of error message you mentioned above. Here's what I mean: