∵m-n2=1,即n2=m-1≥0,m≥1,
∴原式=m2+2m-2+4m-1=m2+6m+9-12=(m+3)2-12,
则代数式m2+2n2+4m-1的最小值等于(1+3)2-12=4.
故答案为:4.