Cannot plot loop between four circle

I want to plot something like the following, loop between four circles
```
+---+  +---+
|{o}+--+{o}|
+-+-+  +-+-+
  |      |
+-+-+  +-+-+
|{o}+--+{o}|
+---+  +---+
```

But ditaa treat it as a cross, it there any options to solve this? I just want a raw line, no arrow on it