```cpp
#include<cstdio>
#include<cstring>
#include<algorithm>
#define N 1001
struct target
{
int x,y;
} t[N*N];
struct queue
{
int x,y;
} q[N*N];
int dicx[4]={0,1,0,-1},dicy[4]={1,0,-1,0};
int n,m,cnt,x1,y1,x2,y2,head,tail,ans=100000000;
int a[N][N],dis1[N][N],dis2[N][N];
bool f[N][N];
using namespace std;
inline int read()
{
int x=0,f=1;
char ch=getchar();
while (ch<'0'||ch>'9')
{
if (ch=='-') f=-1;
ch=getchar();
}
while (ch>='0'&&ch<='9')
{
x=x*10+ch-'0';
ch=getchar();
}
return x*f;
}
void bfs1(int x,int y)
{
head=0;
tail=1;
f[x][y]=1;
q[1].x=x;
q[1].y=y;
while (head<tail)
{
head++;
int nx=q[head].x,ny=q[head].y;
for (int i=0; i<4; i++)
{
int xx=nx+dicx[i],yy=ny+dicy[i];
if (xx<1||xx>n||yy<1||yy>m)continue;
if (f[xx][yy]||a[xx][yy]==1) continue;
dis1[xx][yy]=dis1[nx][ny]+1;
q[++tail].x=xx;
q[tail].y=yy;
f[xx][yy]=1;
}
}
}
void bfs2(int x,int y)
{
memset(q,0,sizeof(q));
memset(f,0,sizeof(f));
head=0;
tail=1;
f[x][y]=1;
q[1].x=x;
q[1].y=y;
while (head<tail)
{
head++;
int nx=q[head].x,ny=q[head].y;
for (int i=0; i<4; i++)
{
int xx=nx+dicx[i],yy=ny+dicy[i];
if (xx<1||xx>n||yy<1||yy>m)continue;
if (f[xx][yy]||a[xx][yy]==1) continue;
dis2[xx][yy]=dis2[nx][ny]+1;
q[++tail].x=xx;
q[tail].y=yy;
f[xx][yy]=1;
}
}
}
int main()
{
m=read();
n=read();
for (int i=1; i<=n; i++)
for(int j=1; j<=m; j++)
{
a[i][j]=read();
if (a[i][j]==4)
{
t[++cnt].x=i;
t[cnt].y=j;
}
else if (a[i][j]==2)
{
x1=i;
y1=j;
a[i][j]=0;
}
else if (a[i][j]==3)
{
x2=i;
y2=j;
a[i][j]=0;
}
}
bfs1(x1,y1);
bfs2(x2,y2);
for (int i=1; i<=cnt; i++)
{
int nx=t[i].x,ny=t[i].y;
if (!(dis1[nx][ny]+dis2[nx][ny])) continue;
ans=min(ans,dis1[nx][ny]+dis2[nx][ny]);
}
printf("%d\n",ans);
}
```
$\rm Pascal$代码：
```pas
const dx:array[1..4]of longint=(-1,1,0,0);
dy:array[1..4]of longint=(0,0,-1,1);
var n,m,x1,y1,x2,y2,t,i,j,min:longint;
a,c,d:array[1..1000,1..1000]of longint;
b,e:array[1..1000000,1..2]of longint;
f:array[1..1000,1..1000]of boolean;
procedure bfs1(x,y:longint);
var nx,ny,k,head,tail:longint;
begin
head:=0;
tail:=1;
f[x,y]:=true;
e[1,1]:=x;
e[1,2]:=y;
while head<tail do
begin
head:=head+1;
for k:=1 to 4 do
begin
nx:=e[head,1]+dx[k];
ny:=e[head,2]+dy[k];
if (nx>0)and(nx<=n)and(ny>0)and(ny<=m) then
if a[nx,ny]<>1 then
if not f[nx,ny] then
begin
tail:=tail+1;
e[tail,1]:=nx;
e[tail,2]:=ny;
f[nx,ny]:=true;
c[nx,ny]:=c[e[head,1],e[head,2]]+1;
end;
end;
end;
end;
procedure bfs2(x,y:longint);
var nx,ny,k,head,tail:longint;
begin
head:=0;
tail:=1;
f[x,y]:=true;
e[1,1]:=x;
e[1,2]:=y;
while head<tail do
begin
head:=head+1;
for k:=1 to 4 do
begin
nx:=e[head,1]+dx[k];
ny:=e[head,2]+dy[k];
if (nx>0)and(nx<=n)and(ny>0)and(ny<=m) then
if a[nx,ny]<>1 then
if not f[nx,ny] then
begin
tail:=tail+1;
e[tail,1]:=nx;
e[tail,2]:=ny;
f[nx,ny]:=true;
d[nx,ny]:=d[e[head,1],e[head,2]]+1;
end;
end;
end;
end;
begin
readln(m,n);
for i:=1 to n do
begin
for j:=1 to m do
begin
read(a[i,j]);
if a[i,j]=2 then
begin
x1:=i;
y1:=j;
a[i,j]:=1;
end;
if a[i,j]=3 then
begin
x2:=i;
y2:=j;
a[i,j]:=1;
end;
if a[i,j]=4 then
begin
t:=t+1;
b[t,1]:=i;
b[t,2]:=j;
end;
end;
readln;
end;
bfs1(x1,y1);
fillchar(f,sizeof(f),false);
fillchar(e,sizeof(f),false);
bfs2(x2,y2);
min:=maxlongint;
for i:=1 to t do
if (c[b[i,1],b[i,2]]<>0)and(d[b[i,1],b[i,2]]<>0) then
if c[b[i,1],b[i,2]]+d[b[i,1],b[i,2]]<min then
min:=c[b[i,1],b[i,2]]+d[b[i,1],b[i,2]];
writeln(min);
end.
```