Hello Doris, I'm Dr. Deb and will do my best to help you (although I'm sorry that you had to wait for a response).
Since your Dobe is such a large dog, this amount of Magnesium shouldn't cause any issues for him. In fact, we would treat dogs with hlow magnesium levels at much higher dosages than this; therefore, this wouldn't be a toxic dose for him.
Overdosages can cause signs (some fairly significant) such as muscular weakness, bradycardia (low heart rate), hypotension, hypocalcemia, respiratory depression and ECG changes. Very high magnesium levels may cause neuromuscular blocking activity and, eventually, cardiac arrest.
...but he would need to have ingested much, much more than 400 mg for these signs to be seen.
He should be fine although, again, my apologies that you didn't receive reassurance sooner. Deb