And were you sending a fraction of a large input, or a combination of lots of small inputs. Both transaction 3 and transaction 4 look like you were sending a combination of lots of small inputs, combined with a large fee. It got confirmed quickly because of the large fee.
What you really need to do is standardize the fee - make them all 0.0001, and vary the amounts (outputs made up of one big input or lots of small inputs). That way you will be able to see whether the miners make you wait or not.
Also, the protocol says you can send an aged input for zero fee if the kilobytes are low. According to the wiki, "a transaction was safe to send without fees if these conditions were met:
It is smaller than 1,000 bytes.
All outputs are 0.01 BTC or larger.
Its priority is large enough"
Test it out and see if it works. Bitcoiners have spent the last seven years telling everyone that you can send "free" or for a minimal fee - so that is what most bitcoiners do. The question is, is that still true.
I sent a combination of a lot of small inputs in each case. Correct 3 and 4 were larger than the others, with more data of similar size inputs. The others were less inputs, with fees that were comparably smaller than the difference in size/number of inputs.
Great suggestion, there are a few ways I could change it up to get more accurate data. Next time I do this (if I can ever find the time to do it right) I'll incorporate that and some other ideas to make the test more scientific and thorough.
Those are great things to keep in mind and I'll refer back to this next time I run a test to remind me the kind of things I should keep in mind.