Well, i'll give a little explanation about the items i've choose for this build.
Hexdrinker: Well Hexdrinker is a very good item that few people use. It's magic resist and damage, plus the shield it gives, makes Jarvan very hard to kill early game, so i'll build it fast.
Philosopher stone: Well, nothing much to say about it, gives mana regen and hp regen, and gold per/sec, so make Jarv stays longer in lane and earning more gold.
Frozen Mallet: Gives a lot of health and slow per hit, wich allows you to always make enemies slowed, plus with your shield and Randuin's it's almost impossible for someone to move.
Randuin's Omen: Armor, Health, Health regen, nothing much to say about this item, and he's active aoe slow, makes a good combo with jarvan skills and previous itens.
Banshee's Veil: Magic Resist and a shield that blocks any incoming ability cast upon Jarv. Prevent some damage and/or CC, give health and mana (both very useable for Jarv) and magic resist. PS: If you see that their team doesn't have that much CC, or they are not focusing you, you can swap banshee's for Force of Nature.
Berserker Greaves: Movement Speed, Atack Speed, good to combo with your Demacian Standard, so you'll have a lot of ASPD, decreasing the chance of people run away, because of your mallet. PS: If you notice that you need some more Magic Res. and/or the other team have a lot of CC, you can change for Mercury Threads, without changing too much on the idea of the build.
BloodThirster: Well, nothing much to say here, as Jarv farm very easily, you can stack BT fast, giving you a lot of damage and life steal, making you stay longer in fights and/or in lane.

As you can see, i'm not build an In Dept guide about Jarvan IV, i'm only sharing with you guys the build i use as Jarv, and many of you maybe never heard about me, but most of people that play with/against me says i am one of the best Jarv they ever seen, and asked me to share my build with them. So here it goes. Notice too, that all items can be changed for some others to make yourself more suitable for that match. For example, you can change Hexdrinker for Force of Nature, and Be with FoN and Banshee's, and/or changing BT for Infinity edge, knowing you'll lose the life steal, but getting a burst on damage. Notice too, that Jarv is not very fast, and instead of the combo Demacian Standard + Dragon strike, you have no other way of running away from battle, so decide wisely when to get in/out a team fight, and when/how to use your skills. That may guarantee your kills and life, but can be the cause of your defeat. Or if you feel you're hittin badly on tanky/tankers champions, you can change hexdrinker or Bloodthirster for Last Whisper, wich is really viable, but make you more squishie.

For this build, as i'm looking forward to give you burst damage + high survivability, i go with:

Greater Mark of Desolation: Armor penetration for a champion as Jarv makes you hit harder then almost anyone in the game. With the high damage plus the passive, you land a lot of damage, even on tanky champions.
Greater Seal of Resilience: Armor, as you intend to be off-tank or a damage burst tanky.
Greater Glyph of Warding: Magic Resist. Same reason as the seal.
Greater Quintessence of desolation: Combined with the mark and your burst damage, be ready to be feared on the summoner's rift.
Greater Quintessence of Vitality, Resilience and Warding: Well that one you can use if, and only if, you're still feeling a little squishie, and feels you're dying to easily on the team fights, but is something i doubt.

PROS

Mix a lot of damage and high survivability.
Gives you a lot of Crowd Control, making you annoying as hell on team fights.
Even against tanks, with your high damage plus high survivability makes you a very good tank killer, even with thornmail.

CONS

Well, i don't see too much Cons for this build, except that you do not give much damage as a DPS JarvanIV, and do not tank as much as a Tank JarvanIV,and you're not very fast, making you harder to flee from battle or ganks. BUT, just for the record, it makes yourself much more suitable for almost all matches.

Well just to end this (i know, i already written too much) i say to you that Jarv is My main, and i'm very good with him (duur you already said that), i'm not doing this build to be the most commented or voted or some of that ****, but to help some people on how to start playing Jarvan IV, as he is not very seen on the field of justice, and mostly, not very well played.

Hope you enjoyed this little guide, and feel free to comment. Even with compliments or saying my build sucks. ;)