May I suggest you perform a few speed checks over a few days..logging the results. You may be paying for 4GB but getting nothing like that and as previous posters have said, irrespective of the system finally chosen, buffering is down to speed in the main.[extreme overload as with the current major rugby matches etc can be another reason, obviously]
I was with Cytanet paying for then 3GB and getting a paltry 1.75GB despite a renewed router...went over to PrimeTel and now get pay for 8GB for €10 less and get 7.50 most of the time and over 8 on numerous occasion.
This is not meant to be a an advert for either supplier, just an illustration on how low the supply might be over the one advertised. Even people living next door to each other using the same supplier might have differing readings, depends on the route to your property.
If you have a poor service.. present your logged findings to them, ask for an engineer to check and a discount to boot.
The latter certainly wakes them up.
http://www.speedtest.net/