Removing higher order terms

You can use a variation of the idea I gave here:

Normal @ Series[
    ss /. {f:u1|u2 -> (s f[#1,#2,#3,#4]&)},
    {s, 0, 3}
] /. s->1

u2[x, y, z, t]^3 + Derivative[0, 0, 0, 2][u1][x, y, z, t] + (3*λ*Derivative[0, 0, 1, 0][u1][x, y, z, t]^2*Derivative[0, 0, 2, 0][u1][x, y, z, t])/2 + μ*Derivative[0, 0, 2, 0][u2][x, y, z, t]