how to calculate the envelope curve of 4 different waves?

Here is a crude function that should do the job, taking into account the fact that your wave scaling is negative.

function env4(wave1,wave2,wave3,wave4,minX,maxX,numPoints)
	Wave wave1,wave2,wave3,wave4
	Variable minX,maxX,numPoints
	
	Make/O/N=(numPoints) envWave
	Make/O/FREE/N=(4)/WAVE wp={wave1,wave2,wave3,wave4}
	Make/O/FREE/N=(4) startV,endV
	
	SetScale/I x (minX),(maxX),"", envWave
	Variable i,j,xx,dx,maxValue
	
	for(j=0;j<4;j++)
		startV[j]=DimOffset(wp[j],0)
		endV[j]=DimOffset(wp[j],0)+DimDelta(wp[j],0)*(DimSize(wp[j],0)-1)
	endfor
	
	xx=minX
	dx=DimDelta(envWave,0)
	for(i=0;i<numPoints;i+=1)
		maxValue=-1
		for(j=0;j<4;j+=1)
			wave w=wp[j]
			if(xx>=min(startV[j],endV[j]) && xx<max(startV[j],endV[j]))
				if(w(xx)>maxValue)
					maxValue=w(xx)
				endif
			endif
		endfor
		envWave[i]=maxValue
		xx+=dx
	endfor
End

You can invoke this on the command line using:

env4('Peak 00','Peak 10','Peak 20','Peak 30',8,-1,500)

Edit:

If you are actually seeking the sum of all the waves then the code is even simpler:

function sum4Waves(wave1,wave2,wave3,wave4,minX,maxX,numPoints)
	Wave wave1,wave2,wave3,wave4
	Variable minX,maxX,numPoints
	
	Make/O/N=(numPoints) sumWave=0
	Make/O/FREE/N=(4)/WAVE wp={wave1,wave2,wave3,wave4}
	Make/O/FREE/N=(4) startV,endV
	
	SetScale/I x (minX),(maxX),"", sumWave
	Variable i,j,xx,dx,maxValue
	
	for(j=0;j<4;j++)
		startV[j]=DimOffset(wp[j],0)
		endV[j]=DimOffset(wp[j],0)+DimDelta(wp[j],0)*(DimSize(wp[j],0)-1)
	endfor
	
	xx=minX
	dx=DimDelta(sumWave,0)
	for(i=0;i<numPoints;i+=1)
		maxValue=0
		for(j=0;j<4;j+=1)
			wave w=wp[j]
			if(xx>=min(startV[j],endV[j]) && xx<max(startV[j],endV[j]))
				maxValue+=w(xx)
			endif
		endfor
		sumWave[i]=maxValue
		xx+=dx
	endfor
End

I hope this helps,

A.G.

Perfect!, Thank you