The most significant difference lies on the number of the foils and blades on its head. Arc4 has four arrays of foil and blade whereas Arc3—obviously—has only three arrays. Although this appears to be a significant difference, it doesn’t actually hurt the performance a lot. You can still achieve fast, close and smooth shave with this shaver, though you may need more time to deal with one particular spot.
The motor that moves its blades is slightly weaker, with capability to output only 13,000 cycles per minute compared with Arc4’s 14,000-cpm motor. However, many of its competitors include motor that can only generate 10,000 cycles per minute, so in general, the motor of this shaver is still considered above the par.
Without mentioning the price difference, which is very obvious for a lower-grade model, other differences are purely cosmetic. You will see some variances in color, design, and LCD display. These are not big deals and ultimately, the reason why you buy this shaver is because it is cheaper.