Splitting string of a textbox

Hello, Obi,  

There is no automatic way for splitting the text with emails in different groups and assigning them to different RadTextBox controls. 

However, you can split the long string by the specific separator and you can count how many emails you have in the array in order to determine what part of the array is 1/4 of the whole text. Then, a subset of the obtained array of text values can be concatenated by the String.Join method and thus assign different part of the array to the different text box controls:
https://docs.microsoft.com/en-us/dotnet/api/system.string.split?view=netframework-4.8 
https://docs.microsoft.com/en-us/dotnet/api/system.string.join?view=netframework-4.8 

I would suggest you to use a RadAutoCompleteBox which would be suitable for displaying emails. Each tokenized text block is separated by character, specified by the Delimiter property. You can change the Delimiter at any time. Please refer to the following help articles which will get you familiar with the RadAutoCompleteBox control and how it displays the tokenized strings:
https://docs.telerik.com/devtools/winforms/controls/editors/autocompletebox/autocompletebox 
https://docs.telerik.com/devtools/winforms/controls/editors/autocompletebox/getting-started 

I hope this information helps. If you need any further assistance please don't hesitate to contact me.

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik