Or try this
eqs = {
D[b1[x], x] == -aa b1[x] b2[x] + bb b3[x] - gg N1,
D[b2[x], x] == aa b1[x] b2[x] - ee b2[x] - dd b2[x],
D[b3[x], x] == dd b2[x] - bb b3[x],
D[b4[x], x] == ee b2[x] + gg N1
}
ic = {
b1[0] == 1000,
b2[0] == 3,
b3[0] == 0,
b4[0] == 0
}
Manipulate[
vals = {
aa -> aaa ,
bb -> bbb ,
gg -> ggg,
ee -> eee ,
dd -> ddd ,
N1 -> 1000
};
sol = Flatten[
NDSolve[
Join[eqs, ic] /. vals,
{b1, b2, b3, b4},
{x, 0, 30}, MaxSteps -> Infinity
]
];
fb1 = b1 /. sol;
fb2 = b2 /. sol;
fb3 = b3 /. sol;
fb4 = b4 /. sol;
Plot[{fb1[x], fb2[x], fb3[x], fb4[x]}, {x, 0, 20},
PlotRange -> {0, 1000}],
{{aaa, .004}, .0001, .01},
{{bbb, .01}, .001, .1},
{{ggg, .01}, .001, .1},
{{eee, .02}, .001, .1},
{{ddd, .6}, .01, 1}
]