Johnson added: "It was a big disappointment to go out of the Cup the way we did.
"But we showed our character at Arsenal - even though we probably saw it as two points dropped after being 2-0 up.
"There have been times when our performances have been better than the points we have been getting.
"But our form in the league has been a lot better recently than it was at the start.
"There are probably six teams fighting for fourth place this season, it's very open. We know it will be tight.
"But if we could get into the top four it would be a massive boost for this club.
"People thought we had no chance at the start of the season but it's a realistic target for us now.
"If we can go on some kind of run until the end of the season, we'll be close."