[Howard Klepper]

What you should be getting by now is that both the #4 and #5 can do your jobs of thicknessing and jointing. It becomes somewhat of a matter in one's taste in how to work, and they are not radically different, but the difference in length is significant.

While with practice you can joint the top with either plane freehand, a shooting board will help a lot and make the #4 work just as easily as the #5 for jointing.

Which one to reach for for thicknessing depends on what kind of surface you are starting with and how much excess thickness you have. It also depends on how comfortable you are pushing a wider plane (such as the 4-1/2). The shorter plane will take out local variations that the longer one might glide over, while the longer one will be a little more consistent from one end of the board to the other. Again, either one can work well, and the difference is not radical.

You don't want to hear about new high-end vs. old iron, but it's kind of hard to divorce it from your question. You could shop on Ebay and get 4 or 5 nice pre-1960s Stanley planes for the price of one Veritas or Lie Nielsen jack. That would make you far less invested in deciding on the one right plane to buy, because you would have 4 or 5 different sizes. I'm just as happy working with a Stanley as with a Lie-Nielsen or Veritas. The latter have some advantages, but I don't see any difference in the result, and one certainly feels more soulful when using an old plane one has fettled (it's not that hard) and one's hands are on the Brazilian rosewood tote and knob.

BTW, don't leave out the #5-1/4 junior jack--a very versatile plane (it is shorter and narrower than a #5, contrary to what the number would suggest. Or the same width if its the Veritas.). I think if I were to have one plane for doing the jobs you want to do, that would be it.