I am planning to stay in the UK for 4 months after virus; I will need to show proof of accommodation to the immigration officer. What are accepted methods to provide proof of accommodation? Can I use a screenshot of my work phone? How many days of accommodations do i usually need to prove? If I will be couchsurfing for 80 percent of my stay in the UK, how should I present it to the immigration officer?
I wouldn't depend on being able to show a screenshot stored on your phone, things can go wrong (phone dies, etc.). You should always print these booking confirmations from the website you booked on.
There's no single answer to the next part of your question, and really depends on the discretion on the officer. If you have some accommodation booked, show proof of it. You'll also need some bank statements (again, printed) and/or credit cards with you to show you will have sufficient funds for the duration of your stay and explain you'll be couchsurfing.