Awhile back I looked at the tier 5 set for bears. And even though the 4/5 tier 4 bonus of 1400 armor is great for tanks, the tier 5 stats are good enough to outweigh losing the bonus. So Tier 5 is good for bears, how about for cats? Turns out it's not so good; the 2/5 tier 4 bonus is too amazing to pass up.
I looked at tier 5 cat gear with Toskk's kitty points analysis. Here's the results I came up with for my T5ish stats (3300 AP, 41% crit). All gear values ignore set bonuses.
T4 T5 Diff
Head 255 259 4
Shoulders 182 217 35
Chest 236 255 19
Hands 143 158 15
Legs 189 265 76
Total 1015 1154 149
So ignoring set bonuses, the T5 is a 15% upgrade over T4. Not bad. But how about those set bonuses?
T4 T5 Diff
2/5 Tier 4 364 0 -364
4/5 Tier 4 68 0 -68
4/5 Tier 5 0 101 101
The 2/5 T4 bonus is so enormous that you can't afford to lose it. In fact, it's good for another 7% DPS all by itself. Not hard to understand why, either: you get a 4% chance for an extra 20 energy on every melee attack. Extra energy = more shreds = more DPS and combo points.
So if you had to choose between T4 and T5 gear, your optimal choice for the 2/5 bonus is T4 head and hands. But the 4/5 T4 bonus is pretty good too; 30 strength is a lot. So really you want to wear T4 everything except for T5 pants. That's pretty crazy.
Fortunately cats aren't limited to T4 and T5 sets, we have a lot of other gear choices. But even with all the options I'm guessing I'll be wearing 2 pieces of T4 for a long time.