The Audi A4 is a very nice sedan, and between the two, purely going on product merits, we would recommend it over the Hyundai Elantra. It delivers a comfy ride and enough space and comfort, the interiors are well built and have a quality feel to them, and the engine too is very refined and delivers a strong performance. However, buying them used means you must bear in mind that the A4 is an older car and with the higher mileage too, plus being a segment above the Elantra, expect to have to spend more on service costs.