I give my T a 2 page typed letter once every 2 or three months, sometimes more often. I will usually hand it to him at the beginning of session but I have emailed them the night before if it involves things that he has done to upset me. That way he can read it before session and have a head's up on how to respond. My T always thanks me for these letters because it gives him information about what is going on in my head. I don't know if I would send a 4 page letter. He would probably not get through discussing one that long in one session. 2 pages for my T usually take almost the entire session and sometimes more to deal with. Maybe write out everything you want to say and cut the initial letter down a bit. Then write up the rest for the next session. That way he can give you more thorough responses to your thoughts. Unless you really don't want a response, then I wouldn't see a problem with a 5 page letter. I know my T always wants to address most of my issues whether I care to talk about it or not.

Your therapist is being paid. It's not a coffee date. It's not your job to be entertaining. The important thing is being able to use the sessions for therapeutic benefit, not having pleasant conversation. If having your therapist read the letter would help therapy be more beneficial for you, go for it.

I regularly (like every week or two) have my therapist read several written pages, college ruled paper with fairly small handwriting.
It's been really helpful because it gets the topic and things I need to say out there when I struggle to actually say them. After a few sessions in which I didn't do that and had a much harder time talking he actually suggested it as something I could do again because it had been helpful.

It helps him when I actually tell him what's going on. And letters are the way I'm able to do that.
Your therapist should want to help you, and any good therapist would be glad if there's something like writing a letter that will help them help you better.

I've found it helpful to write out things to tell my therapist. I think the longest I ever sent her on email was about three pages. But it helped get stuff out there that I just wasn't verbalizing.