Hi User,
I wouldn't worry about having to replace all the elements at the same time unless you are planning to buy an oven with SiC elements. SiC elements change electrical resistance over time and are typically wired in a parallel configuration which is why when it comes time to replace elements they all have to replaced instead of just one. MoSi2 heating elements maintain their electrical resistance and are typically wired in a series circuit and can be replaced individually making them much less costly if one goes out. Whether it be an oven with 4 or 6 elements the size will dictate the efficiency and capacity. Furthermore, make sure you are getting an oven with Speed MoSi2 elements with a maximum surface temperature of 1850C, these have a better surface coating making them less likely to be subject to oxidation from contaminates. These type of elements typically will have a higher heat rate capability 70-100c/min. They are also going to last a lot longer than their lower end regular speed (Max 30C/Min heat rate) MoSi2 counter parts that typically have a maximum surface temperature of about 1700C. They will definitely outlast SiC elements as well as since their maximum surface temperature tends to be about 1600C! This is why SiC elements go out so often. They are running at full blast all the time. The surface temperature is usually about 70C higher than the temperature in the chamber. Thus if your heating element is at 1600C, your chamber is a 1530C. The actual difference in degrees is of course dependent on the R value of the insulation and the chamber dimensions, but it tends to be around 70C.