#include<bits/stdc++.h>
#include<ext/pb_ds/priority_queue.hpp>
#include<ext/pb_ds/assoc_container.hpp>
#ifdef ONLINE_JUDGE
	#define getchar()(p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++)
	char buf[1<<21],*p1=buf,*p2=buf;
#endif
using namespace std;using namespace __gnu_pbds;int read(){int num=0,f=1;char c;while(!isdigit(c=getchar()))if(c=='-')f=-1;while(isdigit(c))num=num*10+(c&15),c=getchar();return num*f;}void write(int x,char ch=' '){int F[20],cnt=0;if(!x)putchar('0');if(x<0)putchar('-'),x=-x;while(x)F[cnt++]=x%10+'0',x/=10;while(cnt)putchar(F[--cnt]);putchar(ch);}
namespace Main
{
    const int N=300010,inf=1e9;int n,k,L,R,rt,A,B,f[N],g[N],h[N],dfn[N],low[N],ti,id[N],dep[N],fa[N],head[N],to[N<<1],nxt[N<<1],tag[N<<2],pl[N],pr[N],tot,x,y,z,xx,yy,zz;vector<int>b[N];__gnu_pbds::priority_queue<int,greater<int>,pairing_heap_tag>q[N];void link(int x,int y){to[++tot]=y,nxt[tot]=head[x],head[x]=tot;}
    void dfs0(int x){int mn=x,se=inf;dep[x]=dep[fa[x]]+1,dfn[x]=++ti,id[ti]=x,b[dep[x]].emplace_back(dfn[x]),q[x].push(x);for(int i=head[x],y;i;i=nxt[i])if((y=to[i])!=fa[x]){fa[y]=x,dfs0(y);for(;dep[q[y].top()]-dep[x]>B;q[y].pop());if((z=q[y].top())<mn)se=mn,mn=z;else se=min(se,z);}low[x]=ti,g[x]=mn;for(int i=head[x],y;i;i=nxt[i])if((y=to[i])!=fa[x])h[y]=(q[y].top()==mn?se:mn),q[x].join(q[y]);if((y=dep[x]+A-1)<=n)pl[x]=lower_bound(b[y].begin(),b[y].end(),dfn[x])-b[y].begin(),pr[x]=lower_bound(b[y].begin(),b[y].end(),low[x]+1)-b[y].begin();}
    void upd(int k,int l,int r,int L,int R,int z){if(L<=l&&r<=R)tag[k]=max(tag[k],z);else{int mid=(l+r)>>1;if(L<=mid)upd(k<<1,l,mid,L,R,z);if(mid<R)upd(k<<1|1,mid+1,r,L,R,z);}}int qry(int k,int l,int r,int p){if(l==r)return tag[k];int mid=(l+r)>>1;return max(tag[k],(p<=mid?qry(k<<1,l,mid,p):qry(k<<1|1,mid+1,r,p)));}
    void dfs(int x){int mn=inf,se=inf,p1=0,p2=0,t1=0,t2=0;for(int i=head[x];i;i=nxt[i])if(to[i]!=fa[x]){dfs(to[i]);if(f[to[i]]<mn)se=mn,mn=f[to[i]];else se=min(se,f[to[i]]);}f[x]=mn+1,yy=dep[x]+A;for(int i=head[x],y;i;i=nxt[i])if((y=to[i])!=fa[x]){if(pl[y]<pr[y]){t2=t1,t1=y;
    for(int j=pl[y];j<pr[y];++j){xx=id[b[yy][j]];if(f[xx]>B&&dep[xx]>qry(1,1,n,dfn[xx])){p1=p2,p1=y;break;}}}upd(1,1,n,dfn[y],low[y],dep[x]+B-1-(f[to[i]]==mn?se:mn));}
    for(int i=head[x],y;i;i=nxt[i])if((y=to[i])!=fa[x]&&((t1==y?t2:t1)?!(p1==y?p2:p1):(h[y]<=k)))upd(1,1,n,dfn[y],low[y],dep[x]+B);if(t1?!p1:(g[x]<=k))f[x]=0;}
    bool check(){return memset(tag,0,(n+1)<<4),dfs(rt),!f[rt];}
    void main()
    {
        n=read(),rt=read(),A=read(),B=read();if(A<=B)return write(1,'\n');for(int i=n;--i;)x=read(),y=read(),link(x,y),link(y,x);
        dfs0(rt),L=1,R=n;for(;L<R;)k=(L+R)>>1,(check()?(R=k):(L=k+1));write(L,'\n');
    }
}
int main()
{
    const bool base=0,IO=1;int T;if(!base)T=1;else if(IO)T=read();else ios::sync_with_stdio(0),cin>>T;for(;T--;)Main::main();
    return 0;
}