Which non-regular bipartite integral graphs with maximum degree four do not have pm1 as eigenvalues?

Krystyna T. Balinska Slobodan K. Simic Krzysztof T. Zwierzyski Which non-regular bipartite integral graphs with maximum degree four do not have pm1 as eigenvalues? 15-24 2004 286 Discrete Mathematics 1-2 http://dx.doi.org/10.1016/j.disc.2003.11.040 db/journals/dm/dm286.html#BalinskaSZ04