t = int(input())
while t:
t -= 1
n,B,x,y = list(map(int, input().split()))
n += 1
dat = [0]*n
dat[0]= 0
s = 0
for i in range(1,n):
dat[i] = dat[i-1] + x
if dat[i] > B:
dat[i] = dat[i-1] - y
s += dat[i]
print(s)
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define int long long
#define For(i, s, e) for (ll i = s; i < e; i++)
#define FOR(i, s, e) for (ll i = s; i <= e; i++)
#define FORD(i, s, e) for (ll i = s; i >= e; i--)
#define pb push_back
#define vii vector<ll>
#define makep make_pair
#define vpll vector<pair<ll, ll>>
#define fi first
#define se second
#define sii set<ll>
#define pii pair<int, int>
#define rev(c) reverse(c.begin(), c.end())
#define sortf(c) sort(c.begin(), c.end())
#define sortd(c) sort(c.begin(), c.end(), greater<int>())
#define test() \
int test; \
cin >> test; \
while (test--)
#define fast() \
ios_base::sync_with_stdio(false); \
cin.tie(0); \
cout.tie(0);
#define sqr(n) (n) * (n)
#define len(s) (int)(s.length())
#define re0 return 0
#define re return
int const N2 = 1e4 + 7;
int const N = 1e6 + 7;
int const inf = 1e18 + 7;
int const mod = 1e9 + 7;
char const nl = '\n';
// ! ¸,ø¤º°`°º¤ø,¸¸,ø¤º° [ нvмegy ] °º¤ø,¸¸,ø¤º°`°º¤ø,¸ roadтoнυe
signed main()
{
fast();
test() {
int n, B, x, y;
cin >> n >> B >> x >> y;
int sum = 0;
int cur = 0, pre = 0;
FOR(i, 1, n) {
if (pre + x <= B) cur = pre + x;
else cur = pre - y;
sum += cur;
pre = cur;
}
cout << sum << nl;
}
return 0;
}
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