Four weeks is a VERY long timeline for an ale. If you pitch a standard amount of yeast, an average-gravity ale should reach FG in 3-6 days. Bottle three days after that and it should be carbonated well ahead of time. You'll probably even have a week or two left for cold conditioning to drop it bright. If your bottles aren't carbonated after 7-10 days, you're probably storing them at too low a temperature. 75°F or so is ideal.
Using one plastic bottle per batch will let you know when the beers have carbonated, without having to open them.