String username = correct usernme;
String password = Correct password;

Blob headerValue = Blob.valueOf(username + ':' + password);
     String authorizationHeader = 'Basic' +
     EncodingUtil.base64Encode(headerValue);

HttpRequest req = new HttpRequest();
req.setEndpoint('https://ap4.salesforce.com/services/apexrest/Accounts/');
//req.setHeader('Authorization', authorizationHeader);
req.setMethod('GET');
//req.setHeader('content-type', 'application/json');
req.setTimeout(120000);
//req.setBody(jsonBody);
Http htp = new Http();
HttpResponse resp = htp.send(req);
System.debug('resp:'+resp);