A number 04 plane has a couple of advantages. Because it's relatively light it's useful for more delicate planing jobs, like levelling the sawn off edges of a box,
In this particular example the workpiece is prone to tear out, so a closely set cap iron on an 04 plane is an efficient way of getting quality results while working right around the rough sawn edge,
Another benefit of the 04 is if you're a site joiner, shopfitter, boatbuilder, or anyone else who has to
carry their tools to the job. Humping tools up ladders or flights of stairs soon adds appeal to smaller and lighter tools! I believe Paul Sellers fell into this category, and after many years of using only an 04 he'll have found ways of making the tool do whatever he needs.
But for a bench based woodworker the weight of the 04 is less of an advantage. And for the beginner the 04 has one huge negative. This photo shows an 04 next to an 05,
It's clear that the 05 provides a lot more sole length ahead of the cutting edge than the 04. Until you've got a fair bit of experience under your belt this will translate into much cleaner starts to the planing cut. I regularly see beginners (and even intermediate woodworkers) making a bit of a mess when starting the cut, with the back end of the plane drooping fractionally which makes it impossible to get a clean, straight result. It's no co-incidence that when schools taught woodworking they inevitably used 05 planes, they're just easier and more versatile.
But once you've elected to go for the 05
length, you still have the question of the 05 and 05 1/2 for the
width. Personally I prefer the 05 1/2, but it's not something I'd die in a ditch over. I find the extra heft of the 05 1/2 makes for better shooting, and that little bit of extra width is useful for jobs like edge planing doors. But in truth I'd be confident of getting the job done with either an 05 or an 05 1/2.
Here are four options in this length category,
It's interesting to look at the weight. The traditional wooden jack comes in at 2080g, hardly any different to the 05 at 2150g, which in turn isn't all that much lighter than the 05 1/2 at 2550g. But interestingly the real outlier is the Lie Nielsen 05 1/2 at 3270g. In use that really is the odd one out in weight terms, and it's useful to note just how much heavier some premium planes can be.
But it's easy to waste a lot of time fretting unduly over the choice of planes. With enough skill and experience you can get excellent results from almost any plane, however it's also true that
without that skill and experience there's no plane that will magically turn you into a master hand tool worker.
The workshop where I trained had the tradition of only using 07 bench planes. That's what you used for absolutely everything, from big chunky stuff like this,
Through to delicate, fussy little details like this,
So it's probably true to say that instead of anguishing over this plane or that, you'd be better off just getting stuck in and use whatever tool you happen to own, because at the end of the day skill and experience will trump tool choice very time!