Hi Erwin,
Strange that your ulmus didn't went into dormancy by now. It should have. Did you kept it indoors for a while?
Anyway, i cannot say whether or not your tree wil be safe now. It depends on more factors than jus the temprature inside the pot. Trees in good draining mixture that are kept out of the sun and out of the wind kan upstand up to -20 when the soil is dry. However, trees in wet ground in the ful sun and wind can die in temperatures of -4. Any bubblewrap won't do any good in that situation.