Problem with macro/vba aimed at making cells on separate worksheets each update the other

Hi -

I'm new to the forum, and new to VBA. I'm a video editor by trade, but need to use excel for billing purposes.

I've seen some answers to this question on this and other sites, but I think I'm struggling with the syntax here.

I have a multi-sheet document where Sheet1 allows a customer to input information which populates Sheet4, which is an invoice. On Sheet1 B6 is where clients type in the project name, which populates C5 on Sheet4. Also I have a VBA that uses the data from B6 as part of a save-as event that names the file according to client, project, department, etc.

Billing would like to be able to update C5 on the invoice (sheet4) and have it update the B6 cell on sheet1 rather than having to switch to the other worksheet to update that single field. So I would like Sheet1B6 and Sheet4C5 to essentially act like a single cell. Change either one of them and they update the other. Here's the macro I'm working with, but not managing to make work:

(ON SHEET4)

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(ON SHEET1)

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It seems pretty simple, but I'm VERY new to VBA. I want to write it something like:
If Sheet4 C5 changes, then make Sheet1 B6 match Sheet4 C5. If Sheet1 B6 changes, then make Sheet4 C5=Sheet1 B6

I'm using Mac Excel 2011. Can someone help me out here? Point out my syntax error or throw some code my way? Would be greatly appreciated. Thanks!

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