If you ever face HE or Empire with moderate to heavy magic, your army will probably be decimated by turn three. If you face an army with eagles, terradons, fast cav, etc., your army will be almost totally impotent because you won't have enough other models/units to make up for the ones being march/charge blocked.
You won't have any combat res in many situations outside of wounds caused and a banner. Rarely you may get the outnumber against small, elite units. Chariots and flank charges will eat them alive. When over 1000 points of your army is in four relatively small units, your enemy can ignore any of them that don't contain your Lord/BSB and focus fire. If you face an army that includes a dragon, he will fairly handily wipe the board with your knights, and unless you take MoS they will many times be prone to auto-flee.
All in all, I think four units of 7 knights is just asking for trouble. That's just a huge points sink. Even if you don't give any of those units a standard bearer (a mistake IMO) you would be looking at over 1100 points of naked knights. Even if your only hero choice is a single Lord, you've only got about 750 points left for marauders, hellcannons, warriors, horsemen, dogs, etc. to support those knights with. If you include a BSB, now you've only got enough points for (maybe) two units of Warriors or a chariot or two. Wood Elves will shoot you to death, Ogres will kill you too quickly, HE and Empire will magic you to death, Dwarves (and Empire again) will stand and shoot with black powder weapons, VC will bog you down and make the knights impotent, O&G and skaven will try to flank the hell out of you. Dark Elves will try to shoot you to death, which may or may not work, but if not they'll have a good shot at winning just on points denial. Bretonnians can do what your knights can do just as well, plus they'll have ranked units and bowmen to back them up. Just very risky, IMO.