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