HDMI 1.4b is what the system supports, so 4K is indeed supported.  That said, it doesn't really matter what the nVidia GPU supports -- all video on this model is controlled by the Intel GPU aboard the processor, even when the data is rendered by the MX250, which acts as a co-processor. The 10th generation Intel CPU (Intel HD GPU aboard) support 4K fully.  
Hi everyone!I have an Inspiron 5590 (GeForce MX 250) and I am thinking of buying a 4k display. I can't seem to find any information about this laptop supporting 4k via HDMI. Can anyone let me know? Many thanks in advance!
Will an external display on my inspiron 5590 display @ 4k?