Hi. The numbers you see are probably in the context of switches, or what we call turnouts (so that we don't confuse the term with electrical toggle switches). A smaller number means that the diverging route leaves the main axis of the turnout (we call it the through route) at a greater angle than it would for the higher number turnouts. The way you think of it is that the number of through units is under the number of diverging units, as in a fraction or ratio 1/4. A #4 turnout only goes 4 through units of length before it has already diverged one unit. A #8 turnouts has to take the engine 8 units along the major axis before it diverges the same distance of 1 unit. So, the higher numbers tend to be long turnouts that take longer to get the engine away to the side. Their angle is lower so to speak. Also, North American style turnouts only curve from the two swinging point rails up to the frog...where everything crosses at the X. Thereafter the path is straight. That means you can't practically use a regular turnout on a curve...at least, not without a jog somewhere.
There is such a thing as a curved turnout which you can find at Walthers. Also Peco and Atlas have started to offer them. Both routes curve in the same direction, but the "through" route curves quite a bit less. I would guess that one of these types will suit you. You'll have to do some fact-finding to see what radius of curvature they have for their inner and outer routes. I have several #7.5 Walthers/Shinohara curved turnouts on my layout.
You're confusing terminology a little bit here:
Turnouts (Switches) are measured by number. This number matches the angle that the diverging track leaves the main (straight through) track. A number 6 turnout has the diverging route seperating from the straight route 1" for every 6" of distance from the frog (the point where the rails actually cross over one another). A number 8 turnout takes 8" of length to diverge that same 1", and a number 4 would take only 4" to diverge that 1". Generally no. 4's are relegated to yards and other areas where speed is slow, number 6 on up is used on mainlines (at least in the model world... I don't think that anything smaller than a no. 10 or no. 15 is used by a 1:1 railroad)
Curves are measured by their radius (usually in inches, though sometimes the european guys will use mm). The smaller the radiusm the tighter the curve is. Train sets (where everything comes in a box) usually have 15" or 18" curves, which are extremely tight, and generally only work with the trainset equipmen. Many modelers will use 24" (or larger) curves as a minimum on larger layouts.
There is no real "drop in" replacement turnout for a curve, although some come close (I believe that the Atlas "Snap Switches" are rough drop-ins for 18" curves). Both curves and turnouts work better the larger they are -- it is usually a good practice to fit the largest turnout or broadest curve that you can in a given area.
I hope this helps you out hiway. Sorry that I don't have any pics available to help illustrate things.
-Dan
Builder of Bowser steam! Railimages Site