RegionPlot with four parameters

Question

I will try to illustrate my question with an example:

Let us define the RegionPlot for b and y as

ybregion=RegionPlot[Sin[b]^2 + Sinh [y] <= 1, {y, 0, 1}, {b, 0, 1}, FrameLabel -> Automatic]

for 0 <= b <= 1 and 0 <= y <= 1. Moreover, we have 0 <= a <= 1 and 0 <= x <= by^2. Notice that the interval for x depends on b and y satisfying e.g. Sin[b]^2 + Sinh [y] <= 1. Let us define, for example, the function

f = a + (b x)/y

I would like to make a plot such that we can see the cases f=0,1,2, for example, in terms of a and b as an (a,b,f) plot. My problem is to handle with the condition Sin[b]^2 + Sinh [y] <= 1 and with the four variables of f. In my original problem a, b, f are physical parameters so a useful plot should be one without x, y.

Answer 1

Let me offer some possible plots.

First regard, that the x-y area is very limited.

RegionPlot3D[
   0 <= x <= b y^2 && Sin[b]^2 + Sinh[y] <= 1 && 0 <= y <= 1 && 
   0 <= b <= 1 && 0 <= x <= 1, {x, 0, 1}, {y, 0, 1}, {b, 0, 1}, 
   PlotPoints -> 100]

{xmax, parxmax} = 
  NMaximize[{x, 
  0 <= x <= b y^2 && Sin[b]^2 + Sinh[y] <= 1 && 0 <= y <= 1 && 
0 <= b <= 1 && 0 <= x <= 1}, {x, {y, 0, 1/2}, b}]

{ymax, parymax} = 
  NMaximize[{y, 
     0 <= x <= b y^2 && Sin[b]^2 + Sinh[y] <= 1 && 0 <= y <= 1 && 
  0 <= b <= 1 && 0 <= x <= 1}, {x, {y, 0, 1/2}, b}]

Edit Corrected typos, got changed pictures.

See f = 1 depending an a,b,x,y

Manipulate[
 ContourPlot3D[a + (b x)/y, {a, 0, 1}, {b, 0, 1}, {x, 0, xmax}, 
 RegionFunction -> 
 Function[{a, b, x}, 0 <= x <= b y^2 && Sin[b]^2 + Sinh[y] <= 1], 
 Contours -> {0, 1, 2, 3}, ContourStyle -> {Red, Green, Blue, Cyan}, 
 PlotPoints -> 30, Mesh -> False, 
 AxesLabel -> {"a", "b", "x"}], {{y, .4}, 0, ymax}]

Or a plot of f[a,b] and x,y as parameters.

Manipulate[
  Plot3D[{1, a + (b x)/y}, {a, 0, 1}, {b, 0, 1}, 
 RegionFunction -> 
 Function[{a, b, f}, 0 <= x <= b y^2 && Sin[b]^2 + Sinh[y] <= 1], 
 AxesLabel -> {"a", "b", "f"}, PlotRange -> {0, 2}, 
 ClippingStyle -> Opacity[0.2], 
 PlotStyle -> {{Red, Opacity[.2]}, Blue}, Mesh -> False], {{x, .1}, 
 0, xmax}, {{y, .4}, 0, ymax}]