Patric Ostergard reports that it was solved in 1999: a(4)=80. This has been shown in A. Br\"ungger, A. Marzetta, K. Fukuda & J. Nievergelt, The parallel search bench ZRAM and its applications, Annals of Operations Research 90 (1999), 45--63. The entry for A087725 has been extensively revised NJAS