According to reformed theologians, was Barnabas an Apostle? We usually read the position that there were only 12 Apostles + Paul, and that after the Apostolic era, we have no more Apostles at all. Specially I see this among Reformed theologians.

But how do these theologians explain Acts 14:14 and Acts 14:4, where Barnabas (not one of the twelve and, obviously, not Paul) is called an Apostle? (ἀπόστολοι is the same word used in Matthew 10:2, for example)


  4 But the multitude of the city was divided: and part held with the
  Jews, and part with the apostles.
  
  [...]
  
  14 But when the apostles Barnabas and Paul heard of this [...]




Note on the verse 4: in the passage's context, we see in verse 1 that only Barnabas and Paul were there at Iconium as Christians in that specific "apostolic" mission, so, when we read verse 4, is clear that "apostles" there are Barnabas and Paul and no one else.



Another interpretation: There is a theological branch that will call them "Apostles of the Holy Spirit", because they were separated by the Holy Spirit in Acts 13:2 pretty much as the twelve were separated by the Lord at Matthew 10:2. Mainly Dispensationalists will hold on this view. And they believe that the Goly Spirit still separating His Apostles today.
A good answer could deal with this view too.
Reformed theologians understand these verses to be using the word "apostle" in a broader sense than is normally used, because of the similarity of the work of the men in that particular context.  Barnabas and Paul's task at that time was similar, and thus both were working as "apostles," that is, as missionaries and evangelists.  Calvin's analysis of Acts 14:4 is typical:


  When Luke calleth Barnabas an apostle together with Paul, he extendeth the signification of the word farther than unto the chief [primary] order which Christ appointed in his Church; like as Paul maketh Andronicus and Junias excellent among the apostles. But if we should speak properly, they were evangelists and not apostles; unless peradventure because Barnabas was made Paul’s fellow in office, we place them both in like degree of office, so may he be truly called an apostle. (source)


Albert Barnes points out that both were "set apart" for missions in the same manner in Acts 13:


  Barnabas is called an apostle because he was sent forth by the church on a particular message (Acts 13:3; compare Acts 14:26), not because he had been chosen to the special work of the apostleship (source)


Jamieson, Fausset, and Brown similarly explain that the word "apostles" is used here to describe Barnabas "not merely as Paul's companion, but in their missionary character."  At that particular time, before Paul's apostolic authority was exercised, the term could be used more generally:


  In no other character had Paul as yet stood forth among his brethren. His distinctively official apostolic authority had as yet no scope for its exercise. And since in the missionary character of his apostleship there was no perceptible, and hardly any real difference—if any at all—between him and Barnabas, why might not our historian, with propriety enough, style them both "apostles," without implying that there was not, and never would be, any difference between the two as apostles? (source)