The blurriness is more likely from either too little light to resolve edges (remember, it's trying to produce something to 'see' the same way a camera would), or incorrectly selected lighting for interiors.
4K is way higher than necessary for a decent viewing onscreen, especially on a phone, older computer or tv screen - If you're sending to a tv for a presentation, why go bigger than 1080p which the tv is maxed out at, right? I just checked and an 800x450, 39sec render of a fairly dark hallway has crisp, easy to read stones in the wall, thanks to plenty of light. It's not just the resolution that affects how sharp it ends up looking.
