import math
for _ in[0]*int(input()):
 D,A,B,K=map(int,input().split());l,h=-1,A*B*K+2
 if A*B%D<1 and D*K+math.gcd(D,A)>=A:D,h=1,0
 while l+1<h:
  x=n=(l+h)//2;a,b,c,m=B,-(A*B//D),D,A;s=v=d=0
  while n:a+=c//m*b;c%=m;s+=d//m*b;d%=m;v=max(v,s,s+a*~-n+b*(n:=(c*~-n+d)//m));s+=(a+b)*(a<0);a,b,c,d,m=b,a,m,m+~d,c
  if v<=B*K:l=x
  else:h=x
 print(D*l)