Have you considered why he bucks? Horses that buck on the lunge line usually do so in high-spirits or in protest.
I usually give one "free" high-spirits buck, especially to young horses. I am not on them nor am I near them, so I do not really care. After that, however, it is time to work; any more bucking will result in discipline. If your horse has excess energy, evaluate his feed and turnout schedule. It is not uncommon for horses to be made hot by feeding them high-octane feed and limiting turnout.
For protesting bucks, "it depends." Protesting bucks are usually done in protest of human error, such as giving a too aggressive cue, giving poor release, lunging in too small of a circle, or lunging for too long.
Your cue must be in the correct proportion to his needs. For example, some horses are so sensitive that they will increase their speed just from a raise in your energy level; and if you use a whip as a cue instead, they may feel that is too aggressive and buck in protest.
When you ask for an increase in speed, you must release the pressure the moment he complies. Any continuation of pressure nags and confuses the horse, which could lead to bucking.
Lunging is already hard work and lunging in smaller circles and of longer durations only increase the difficulty. If your lunging circle is of good size, he may be bored. How often are you lunging? How long are you lunging? Horses do not understand the concepts of "exercise" and "training". Do not confuse "consistency is key" as "no variations." Vary what you ask of him and where.
Why are you lunging? There are many four-years-old that can be safely started under-saddle.