用线段树模拟一下就好了~ 

code: 

#include <cstdio> 
#include <algorithm>   
#define lson ls[x]
#define rson rs[x] 
#define N 10000006 
#define ll long long 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std;  
ll A,B;
int n,k,tot;           
int ls[N],rs[N],sum[N];  
void update(int &x,int l,int r,int p) 
{
    if(!x) x=++tot; 
    ++sum[x];  
    if(l==r) return; 
    int mid=(l+r)>>1;  
    if(p<=mid)  update(lson,l,mid,p); 
    else update(rson,mid+1,r,p); 
}       
ll query(int x,int l,int r) 
{
    if(!x) return A;   
    if(l==r) return B*sum[x];       
    int mid=(l+r)>>1;     
    ll re=B*1ll*(r-l+1)*sum[x];   
    return min(re,query(lson,l,mid)+query(rson,mid+1,r));    
}
int main() 
{ 
    // setIO("input");        
    int i,j,bs=1,rt=0; 
    scanf("%d%d%lld%lld",&n,&k,&A,&B);     
    for(i=1;i<=n;++i)  bs<<=1;    
    for(i=1;i<=k;++i) 
    {
        int po; 
        scanf("%d",&po),update(rt,1,bs,po);   
    }   
    printf("%lld\n",query(rt,1,bs)); 
    return 0; 
}