This number means the heat range of the plug. The higher the number the plug can operate under higher heat and lower the number the plug can operate under lower heat. Higher heat means very aggresive driving like racing and mountain drive with heavy load, and lower heat means our daily drive in baba mode. What happens if you mismatch
1 Too hot a spark plug, and you will have carbon fouling on your plug. As the hotter the plugs are smaller the ceramic insulator so as run your car in lower heat with hotter plugs the temperature loss will be great, and the mixture wont ignite properly. you can also call that number the index of heat dissipation the higher the number the greater will be heat dissipation. So if a plug with greater heat dissipation is used in a low heat engine application the temperature of mixture will remain below threshold. Every plug has a specific heat range at which it operates its best so a hotter plug i.e a plug with higher numbers i.e 6.7.8 will have higher operating temperatures than that of lower like 3,4,5 thus it will operate at higher heat condition more easily
2 Too cold a spark plug and you will have melted plug electrode, because the colder plug i.e with lower number has bigger ceramic insulator, thus the electrode remains hotter than that of hotter plugs. The already hot electrode when coupled with high heat of combustion chamber will cause the electrode temperatures to rise and electrode will melt. Also a colder spark plug has a lower ideal operating temperature than hotter plugs.
Hot plugs mean the plugs with higher numbers
Cold plugs mean the plugs with lower numbers
This 4,5,6,7 is not universal
Denso use 16,20,22 etc But the order remains same lower the number colder the plug, higher the number hotter the plug. Actually confusing thing is that this hot and cold name drives itself from not only the operating temperature of the plug but also from heat of engine application.
Another thing is that research has proven a change in tip temperature change of around 70-100 C with a change in single grade in heat range. Like moving from 5 to 6 in case of NGK or 16 to 20 in case of denso
I hope I made my point clear.