John,
Tony is correct in that the place to start, when the plans don't specify a recommended size servo... is from the dimensions of the available space that is provided for the servos. Being a 4m wingspan ( ie close to 13 feet )... I'd suspect you have a lot of room and it's probably going to use the standard size servo instead of the smaller ones like what Tony and I will likely be using on our size of models... but that is just a guess.
One of the spec's that I hate seeing mentioned though is only specifying the weight of a servo which tells you little about it's performance specs... I'm not saying weight isn't important, it is especially on the smaller size models... just that it doesn't tell you anything useful like physical size, torque, speed, bearing type, gear type etc.
So just generically speaking....
Once you know what size the model will accept... Next you will need to figure out how much torque it will require so you can select a proper one from those available. The weight of the model will help in determining the torque but probably not as much as you may think... the location of say the ailerons will also determine how much torque they will likely need... A model with ailerons towards the outer tips of the wings will generally require a little less torque than those towards the center of the wing because the ailerons will have more leverage out on the end of the wing. On the elevator... the length of the model from the CoG to the end where the elevator will again determine how much leverage it has/needs... a short coupled fuse will require a little more than a longer airframe.
The types of aerobatics you plan to fly the model will also determine how much torque you will need... higher aerobatic maneuvers will need more torque than slow gentle flights. A glider isn't normally known for higher aerobatics but you will still need enough torque to get you through loops and perhaps even rolls.
Gliders and sailplanes aren't nearly as popular as many of their powered brethren... so finding specific or exact info is going to be a bit more difficult since talk about them on forums will also be more limited. Where I would start... is by looking up information for gliders that are of a similar size... weight is less important because it is more about the leverage it has and torque a control surface will require but should also be considered since it is a factor. When you have located a popular glider with similar specs... look up the manufacturer's website to see if you can find out how strong of a servo they are recommending... at this point you aren't looking for size info but how much torque the servos they recommend require... then take that info and find something similar or stronger in the size servo your model will accept.
When I'm selecting a servo I try to pick one that I will be able to use in other models also. So most of the time, I look for one that is of the size I need and with the most strength and speed I can get in that size. I always try to get one with ball bearings since they will live longer and won't develop slop in them as quickly as the non-BB types. I also look for metal gear types since they won't strip as quickly. Using that method means that I will likely spend more than what the recommended equipment would cost... but I can then also use them later on other airframes and not have to replace them as quickly as I would if I selected plastic case bearings and nylon gears. I still have to take into account the model... for instance a smaller foamy won't always fly as well if I select a heavier servo like those I just mentioned that I prefer.... so you do have to take into account weight in some instances.
The following links are just general info you might find handy...
http://www.bpesolutions.com/productimages/mec.parts/mch2.html
Servo Database - RC Servo Specs and Reviews
Hitec Servo Chart
