Q: iMessage > confusion between send/receive phone numbers

Hi,

I gave my parents my iPhone 4 (to use as a backup/emergency phone) that's associated with my Apple ID.  And I use an iPhone 5.  Both have different phone numbers.  Both are using the latest OS.  Sometimes when I'm iMessaging someone, he/she will get my response from the iPhone 4's phone number...and will be wondering who that is.  In Settings>Messages>Send&Receive, I do have both numbers checked for "I can be reached at..", but only my iPhone 5's number checked for "Start new conversations..." 

This doesn't happen all the time, but appears to happen in the middle of conversations.  Has anyone else experienced this and found a work-around or solution?

Thanks for your replies!