The 5000 points game is going to change a few things...mass ballista is simply not viable in such circumstances...

How can the amount of points affect overall strategy? I don't think it matters.

If mass ballista has been a successful tactic in 4000pt games why will it not be the same case in 5000pt games?

Say, I use proportions when creating the combination of my armies, eg. Marksmen ususally take up 50% of the total points used.

Yes Boe,It's always better to have a well proportioned army.I never go 50% with ranged though because an army should be based around it's infantry.Maybe 30-40% range and 10-15% cavalry.Too much of any one troop type is a definite achilles heel.

The reason why it won't be so successful in 5000 point games is the sheer number of ballista you'd have. Right now, you can just about get a shot out of every ballista (i.e. have a target for them to shoot at in the thick of the battle without having spares).
However, if it gets much higher, you'll start to see wastage. Like will probably be the case in the 5000 point-ers. In which case, go marksmen so you have less wastage (wastage is crucial in battle games).

Tarim - actually, lots of one troops type is supposed to work that way. It defines definite strengths and weaknesses. The key being that you try emphasise your strength so much that your weakness never gets to be played against you. Mass ballista for example, are weak to melee assault - so you use stopgaps (Squires or spearmen) to buy you enough turns to kill the oncoming army.

Of course, it's rarely that plain and simple - other things come into play like scouting and order of combat. But you get the general idea. And mass ranged is not the ONLY strong set-up out there...