After recently found risk for ios 7 device and releasing an update to resolve it, is it a risk to keep ios 6 on iPhone 4?
Yes, there is a risk because you're using unsupported software which is never recommended. That's the reason why Apple gave out iOS 6.1.6
One of them is the SSL leak which is fixed in iOS 7.0.6 and iOS 6.1.6
However, you can also install the SSL-fix by Jailbreaking your device (if you did not do this yet). You can use this guide.
Sidenote: If you have an iPhone 4 and not a 4s, as stated in your question I'd use the Jailbreak Fix. iOS 7 killed my iPhone 4. It's too slow.
Since there is a known vulnerability and Apple won't let you update your iPhone 4 to 6.1.6 (yet) It is impossible to say there is no risk.
So yes there are some risk.