Hi @ GavinW-Work,
Could you please explain more about the path "OD4B/Documents/Updates/Daily" to save the email attachments in your step3?Is it a file path in your Onedrive for business?
I would offer you a work around for your step1,2,3,4 to save the the email attachments to the path in your Onedrive for business.
However,for your step 3a to overwrite the attachment if the attachment is exist in the folder, it's not supported in microsoft flow Onedrive for business connector currently,I afraid that there is no way to achieve your needs in Microsoft Flow Onedrive for business connectorcurrently.
If you would like this feature to be added in Microsoft Flow, please submit an idea to Flow Ideas Forum:
https://powerusers.microsoft.com/t5/Flow-Ideas/idb-p/FlowIdeas
Please refer to screenshot below to create the flow to achieve your step1,2,3,4:


Note:
1. The folder1 is a subfolder in inbox.
There is a limit in the amount of emails that the “Get emails” action retrieves, the “Get emails” action could only retrieve at most top 25 emails once, if your e-mails is not in top 25 emails, the “Get emails” action would not retrieve it.
2. Add a "Create file" action inside the "Apply to each 2", choose the file path in the "Folder Path"field of "Create file" action with folder picker. Choose the "Name" (Attachment name)dynamic content in the "File Name" field,Choose the "Content" (Attachment content) dynamic content in the "File Content" field.
3. Add a "Delete email" action outside and under the "Apply to each 2".
The flow would run successfully to save the email attachments which is sent by "xxxx@xxxxx.com" to the path in Onedrive for business,and then delete the email as screenshot below:

Regards,
Alice Zhang