Well, the first thing I would do is change the batteries in the keyboard, since you have this model. Batteries near end of life can cause strange things to happen.
If that didn't solve the issue I would open up the keyboard case and clean everything. The problem could just be a bit of dirt or rubbish stuck near a key.
Then I wouldn't buy anything until I was sure the problem was gone with the keyboard powered down either. The keyboard has a power button so it is easy to just switch it off when you don't need to use it.
My motto: Repair, Reuse, Repurpose, Recycle, Replace (or Rubbish).
If I did need to replace the K400r I would look at one of those small combined Remote Controls with the tiny keyboard on the back for daily use. But they probably aren't good for typing much, like I am doing with the K400r right now. So I might keep a larger keyboard such as the K400r and have a new small unit as well. But I haven't looked at what is available recently.
PS: Go for quality, not price, on those small units. Some of the cheap units are just that: Cheap.