(Adds details from security source, updates death toll) TUNIS, April 7 (Reuters) - Islamist militants attacked a Tunisian military checkpoint on Tuesday, killing Four soldiers and wounding three others in a raid in a central region bordering Algeria. Tunisian forces have tightened security since gunmen stormed the national Bardo museum in the capital Tunis last month killing 21 foreign tourists in the **rth African country's worst militant assault in more than a decade. ...