No high pressure lbs require a specific ballasts to operate the bulb correctly
A desk lamp which takes a regular size bulb. You could easily change it to a higher watt.
Yes, you can use a regular bulb in a 3-way lamp. You won't get the 3-way effect though, the regular bulb will just turn on and off.
Usually the 60-watt limit is used to prevent the lampshade overheating, so a 75-watt bulb should not be used. But if more brightness is required, try a low-energy bulb because a 15-watt one gives the same light as a 75-watt incandescent (old-style) bulb. Or a 20-watt would be even brighter.
The voltage isn't decided by the lamp, but by the supply the lamp is hooked up to.
6 watt bulb for mini lamp
A 120 volt table lamp with a 75 watt bulb will pull 0.625 amps. With a 100 watt bulb it will pull 0.833 amps. And with a modern fluorescent 13 watt bulb it will pull 0.108 amps.
40 watt light bulb
Yes. Lower wattage would be safe for the lamp wires and components.
If the socket threads of the lamp are the same then yes the bulbs can be interchanged. The 5 watt bulb will glow brighter than the 4 watt bulb. If by a 4 watt light you mean a 4 watt fixture, then it is not recommended to place a larger wattage lamp in a fixture that is rated by the manufacturer at a specific operating wattage
As long as the lamp holder will take the larger wattage lamp and the current of the circuit is sized to take the larger current then yes, the lamps should be interchangeable.
The wattage of a lamp is normally written on it. If you need to work it out you can measure the voltage across the lamp then measure the current passing through it. Multiplying the voltage by the number of amps will give you the wattage.
As per the formula for power (Power (Watt) = Voltage (V) x Current (i) & Resistance (R) = V / i), 25w lamp bulb would have higher resistance compared to that of 5w lamp bulb.