Harpmaker, the short answer is no. By wiring two 4 omh in parallel to a mono 4 omh amp you would show 2 omhs to the amp and might burn it out. You could buy two 8 omh speakers, which would show 4 to the amp and be within limits. Two speakers would not give you more total volume, but because the speakers would spread the volume more evenly, the sound would appear louder throughout the room.

But better yet, get a new amp (4 omh mono? What country are you writing from, Russia?)

As for John from Hull, I am not at all familiar with British makes (other than Celestion, and some Allen & Heath stuff) but if the speakers are not too big, try some form of the following. If you want more bass, put them in a corner for a third regeneration of the bass (both side walls and the floor) or suspend them in the corner of the ceiling (same thing, two side walls and the ceiling). If you want less bass, put them on a padded floor in the middle of the wall. If you want clearer treble set them up on a stand at something approaching seated ear height (tweeters about 4 feet off the ground). If you want something of both, put them on the aforementioned stands in the corners of the room. You then get 2nd regeneration with direct treble. Good luck!