Preserve Existing Text Formatting When Updating / Adding More Text?

I am writing a macro to automate the updating of text records. For example, in an existing file I have a text record (in a single cell) like:

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and in a separate file containing today's updates, the corresponding record says:

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I want to add the new information to the existing record in my existing file (which is pretty simple). But the problem is that I manually highlight certain text in my existing file (in bold red text) like this:

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and when I use a macro to concatenate that existing text with the (new) added text from the other file, my manually-highlighted text gets set back to plain (black) text. So instead of this:

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I get this (red highlighting gone):

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Is there any way for a macro to append new text into an existing cell, while retaining whatever individual font modifications exist within the original text?

Why did you use the Text property (rather than just the length of range B4), and why did you add 1 to the existing length?

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I tried your latest suggestion and I still get the same error message. Running Excel 2016 by the way.
I really appreciate the help, just isn't working for me for some reason!

Ran some more tests and discovered what I think is the root of my problem. My initial example above was simplified. The actual text I am trying to manipulate is >255 characters. I ran a test with less than 255 characters, and got things to work OK. But if the original text is greater than 255 characters, it doesn't work. I'm no longer getting the error 1004 message (???), the macro runs OK but the original text remains unchanged (if it's >255 characters). So looks like I'm out of luck...