Actually there IS a problem.
You will still be running in SINGLE-CHANNEL mode which causes a slight CPU-Memory bottleneck though it depends a lot on the program used and how fast your CPU is.
The best option is to either get another identical 8GB module, or do nothing.
*Be aware that there's currently no video game that will benefit from more than 8GB of System RAM. Video Editing (not simply converting) and some other demanding software can benefit. So if it's just for video games then another 4GB will likely do nothing.
However, we are back to the possible issue of losing some system performance with only one stick. There's lots of varying opinions even among those who have done lots of testing but all I can say is probably somewhere between 0% and 20% depending on the CPU and task but likely at least SOME performance loss.
(Some confusion exists because with 8GB we often see a lot of that used. Windows always tries to buffer as much as it can, within reason, into the faster System Memory in case you might use it. Mainly programs you've used already. However, if it needs the space such as a video game then it frees up the space to load the game.
Some tools still exist that "FREE" up your system memory to increase performance however they aren't a good ideas as Windows does this already quite efficiently.)