Question: I need a noun that includes all of: (desktop computer, laptop, tablet, smartphone).

Back in the day, in English, a "computer" normally only meant a desktop computer. These days, I think most people understand that desktops, laptops, tablets, and smartphones are all "computers", they just have different form factors.

What about Japanese? Does "コンピュータ" mean: desktop, laptop, tablet, smartphone?

Answer: 
What コンピュータ refers to greatly depends on the context. When explicitly asked, most people of course understand that a smartphone is also a kind of コンピュータ. But in daily life, people tend to use コンピュータ to refer to bigger ones, such as desktop PCs or supercomputers. If someone just says "昨日コンピュータを買ったよ", it probably means they bought a Windows PC or a Mac.

One catch-all term you can use in business/formal settings is 情報機器, which usually includes various products such as desktop PCs and tablets, and sometimes facsimiles, game consoles, and so on.
I'd say: **tablets, yes; smartphones, no**.

The word コンピュータ is not actually used all that much, it sounds a bit formal/heavy iron. The everyday word for personal computing devices is パソコン or PC, which by default means a desktop. [Subcategories of パソコン](http://www.yodobashi.com/%E3%83%91%E3%82%BD%E3%82%B3%E3%83%B3/ct/11970_500000000000000201/) include laptops (ノートパソコン or just ノート) and tablets (タブレットPC).

Mobile phones ([携帯電話]{けいたいでんわ} or [携帯]{けいたい}), on the other hand, are a [different category](http://www.yodobashi.com/ec/feature/174101_000000000000024018/index.html) altogether, further split into featurephones/dumb phones ([ガラケイ](https://en.wikipedia.org/wiki/Gal%C3%A1pagos_syndrome)) and smartphones (スマートフォン, スマホ).

All that said, while this split made a lot of sense ten years ago, with today's devices the line is increasingly blurry, particularly for tablets. For example, while Kakaku.com's categories follow the Yodobashi split above, if you [search for "iPad"](http://kakaku.com/search_results/ipad/) you get back 70k hits under パソコン and 50k under 携帯.