Would it be okay to say "too" instead of "so" in this context?
Answer from a Native speaker
No, "too" doesn't quite fit in this context! "Too" here would mean "over a limit." The use of "so" here emphasises that four days feels like a long time, not that four days is over a certain limit. Here, the function of "so" is more similar to "forever" or "such a long time." It emphasises the dramatic feeling of time taking a while to pass. Ex: Four days have never felt like such a long time before. = Four days have never felt like forever before. Ex: You were taking too long at the shops, so I left.