According to Microsoft, Windows 10 requires a 1 GHz or faster CPU.
Apart from this the CPU should support PAE (Physical Address Extension), NX (No-eXecute) and SSE2.
The Intel i5 4200U supports all these instructions and you can validate it by running CPU-Z. Only thing is that Intel's way of saying its CPU supports PAE and NX is to look for the EM64T and VT-x/VT-d instruction respectively is present or not in the set of intrusions listed by CPU-Z.
I am posting my i5 4200U's CPU-Z screenshot for your convenience.
![enter image description here](../../I/static/images/29a883019d8c5aac02b8970852b7f3712a1a49335bb25325ec2ed0f751742645.jpg)
So you are good to try Windows 10.
1Windows 10 would definitely work with this processor.
However, if it is saying it is not supported, I would recommend speaking to Microsoft, perhaps they may have more of an idea. – Elliot Harrison – 2015-06-16T08:40:31.793