Generally, yes, the 1080p content ends up blurrier on a 4k TV than it should, despite a 4k TV having an even pixel scaling ratio of 4:1. Clarity will also depend on scaling hardware, if any, and other video processing being done on the 4k TV. On moving video content, you probably won't notice the blurriness, but on static displays such as computer, game console, or device menus you might get from a set top box you can. Some models of 4k TV have absolutely atrocious input lag that can't be fixed, so I recommend attaching a game device to the TV for testing before purchase if possible, if you plan to attach the TV to a computer or gaming console once in your home.
Not all HDMI inputs on 4k TVs are HDMI 2.0a and HDCP 2.2 compliant. If your equipment requires those, make sure the TV you plan to purchase has enough to meet your needs.
Another metric to consider is that, to actually benefit from 4k resolution when watching 4k content, you need to be closer to an equivalent sized set than you do with 1080p content. It's simply how human vision works, and if you're not going to be close enough to even appreciate the benefit a 4k TV of the size you purchase might provide, you can save a lot of money by not going 4k, or perhaps step up to a better 1080p screen for a similar price.
Here are a couple of pages that can help you determine the optimal viewing distance for a given size of TV, or help you determine the right sized screen for a given viewing distance:
https://www.tekrevue.com/tv-screen-size-calculator/
http://www.crutchfield.com/S-WfwvZVx1q1g/learn/learningcenter/home/TV_placement.html