T4 has a spell check feature as part of its text editor. You should always run a spell check before saving updates to your pages.
If, while running a spell check, you need to add a word to the dictionary, be sure that the word you add is spelled correctly.
When you add a word to the dictionary, it will be added to all sites to which you have access.
However, the added word will be available to you alone, and not to other users. Other users will not see words you’ve added to the dictionary, even when they have access to the same site(s) as you. Those users will have to add words to their own dictionaries, using the same process.