I wasnt too familiar with the at-4, so thought I would go an check it out.
I like the magazine feeding design, the extra 14 rounds they carry over the rap4's t68 mag ( 11 rounds ) is impressive, but when it comes down to cosmetically speaking ... I think the t68 and rap4 have it hands down, ESPECIALLY the rap4. Though, rap4 IS coming out with a " box mag " which claims to hold 200 rounds ... but until then you would have more rounds per clip with an at4
Not sure how your fields are up there in canada, but here in the US, most fields dont allow .43's an .40 on the field at the same time as the .68 cal's ... might want to check up on that before you buy a rap4 .43 or .40 cal marker.
Something else to consider, the weight of the marker ... the rap4's are 6.14 Lbs, T68's are 6.5Lbs, while the at4 is 5 Lbs, a lil lighter and depending on your style of game you might want to consider going lighter ... but a pound differance isnt a huge differance either lol
One thing the T68 has over the AT4 ... the ability to switch from hopper to magazine fed ... and while your wanting a mag fed marker and can relate to that, I find having the ability to HAVE a 200 round hopper in certain circumstances is essential, that is till rap4 comes out with their box mag with 200 round capacity in which case my " hopper adapter " will be removed permanently for the ability to " patrol " with a mag an switching to a box mag IF/when its needed.
Im not familiar with the accuracy of the at4, so cant say much on comparison between the 3 markers ... but the T68 is pretty d**n accurate and find myself out-shooting most of your everyday PB markers like the tipman 98's, the tipman a5 the alpha m4's ect.
In the end, its really up to you ... but with a $750 US Dollar price tag for their stock at4 in comparison to $500 for the T68 ( stock ) ... go with the t68 an upgrade the heck out of it

. Besides, you can get a refurbed T68 for $350 even, thats $400 in your pocket saved or to be spent on upgrading ( highly recommend upgrading barrel and trigger

.
Hope this helps