What's the difference between a No.4 and No.5 hand plane

Question

Background

Most current and past hand-plane makers seem to follow the numbering system (and design details) popularized by Stanley (and invented by Leonard Bailey?).

There are a bewildering number of different types of plane, but there seems to be a grouping of the general-purpose bench planes used in converting a roughly-hewn chunk of wood into a straight, flat, smooth, square piece of timber:

• fore planes - used first to remove large irregularities.
• jointer planes - used to flatten longer sections of wood.
• smoothing planes - used last to produce a smooth finish.

A No 5 jack plane is often described as a type of fore plane (in some descriptions it is used after a scrub plane). A No 4 is a typical smoothing plane. One or other of these types seem to be most commonly the ones first purchased and used by DIYers.

Question

I have recently bought a second-hand Record No.5 jack plane and was pondering whether I should buy a No.4 smoothing plane. However it seems to me that the similarities are many and the differences few and that I could probably set up the No.5 to do a reasonable job of smoothing.

Similarities

• Same iron angle (45°)
• Same angle of grind on iron (25°)
• Same width of iron (2 in.)
• Same size mouth?
• Same system (identical parts?) of adjustable frog, chip-breaker, lever-cap, etc.

Differences

• Longer (14 in. instead of 9)
• Heavier (due to extra length)

^{Images from old Record company catalogues}

Is there any other physical difference that makes a No.4 more useful for smoothing, other than it's smaller size and weight making it easier to handle?

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Addendum

Following keshlam's answer I found this guide to grinding plane irons which others might find useful

Answer 1

The plane iron may also be ground slight differently in shape (as opposed to angle)... but yes, those are the largest differences. They do affect how the plane is used.

A longer plane improves its ability to accurately flatten a surface (it can level out wider-spaced hills and valleys where a shorter plane would just follow the curves). But depending on what you're doing, the additional weight may be tiring or may be helpful -- harder to lift, but the weight helps keep the iron in contact with the wood.

The ideal hand-tool woodshop has a range of planes from jointers (long) all the way down to block planes small enough to hold in a hand, plus various specialty planes (shoulder planes, rabbeting planes, router planes...) each of them having its own trade-offs -- just as the ideal woodshop has a range of chisel sizes and types. Most of us don't have ideal shops, and that's OK. You can do the same jobs with a smaller setup, it may just take more work and care.

If I had to pick just one of those two, I think I'd have gone for the #4 first... but that may be personal bias; I've already rescued several of that size from various garage sales, and because I haven't had to do serious smoothing/jointing yet. And I'd suggest a block plane as your second purchase -- they're light enough to carry anywhere and more useful for spot-trimming than you might think.

If you do buy a 9" plane, it's seriously worth considering a low-angle plane (lower bedding angle, higher angle on the blade). Among other things, that gives you a wider range available if you need to grind blades to other angles when handling special cases. These do tend to be a bit more expensive, though, largely because this design is mostly offered by the higher-end manufacturers.

(Edit: Yes, I know, "long jointers are long, duh". Redundancy fixed.)