It's still on level 3. You can see it's "3/10"- you must be confusing the 4% bonus effect as the level. 

i see what you are saying,  what do i need to do to get it to level 4

Farm more medals from the arena, upgrade the bonus one more time. :)

Just a little tip though, you'd want to upgrade the accuracy stats first. This makes building for CB a lot easier in the long run. 

Lxzy is correct about your Arena Mission.

Your Great Hall upgrade is only level 3 out of 10 levels.

This is why your mission isn't complete.

You need to get to level 4 out of 10 levels.

If you ever find yourself confused about how many medals you need to complete a Great Hall Bonus, All you have to do is click on the Great Hall Bonus you are working on.

A picture like below will pop up on the right side of your Great Hall Screen:

i

The above picture is a screenshot from my HP Magic Bonus.

I am currently at level 6 with my HP Magic Bonus.

In order for me to get to level 7, I would need 700 Gold Medals.

If you was to click on yours, It would show you at level 3.

In order for you to get to level 4, You need 400 Silver Medals. 

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In addition, Lxzy is correct about your Great Hall upgrades.

You should start with Accuracy first.

It is considered to be one of the best medal efficent options.

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Lastly, I have created a Great Hall Guide for Beginners to Experts.

You can check it out in the below forum:

https://plarium.com/forum/en/raid-shadow-legends/673_guides-and-tutorials/192014_great-hall-guide-for-beginners-to-expert/ 

My Great Hall Guide helps uncover the mystery of the Great Hall.

I have talked with a lot of people who are intimidated by the Great Hall.

The Great Hall can be very overwhelming to people.

They don't know what they should do.

They don't want to make mistakes.

They have so many questions about the Great Hall. 

- What Great Hall Bonus should you do first?

- What Great Hall Bonus should you do last?

And that is why I made my Guide.

@Player J 

You, I've seen you all over the internet helping people on websites from Ayumilove to here. You are a cool dude.

Now I get a chance to help you. The link you posted for the Great Hall Guide doesn't work (I think maybe you left out a character or two). Here's the link if you wish to edit your post.

https://plarium.com/forum/en/raid-shadow-legends/673_guides-and-tutorials/192014_great-hall-guide-for-beginners-to-expert/

Keep up the good work dude.👍

@Andiel Ralck

Yeah, I like helping people when I can.

Thanks for the link

I don't know why mine didn't post.

I probably made an error like you was saying.

I have editted my previous post to put the right link.

Thanks you for your kind words.

Merry Christmas to you.