Congradulations on your graduation from folders to fixed blades. The fixed blade fighting knife is a serious defensive tool and with the proper training can be just as effective as the handgun at extreme close quarters.
In a 4-5" knife for IWB carry I would reccomend the SOG Pentagon. It was my primary carry blade for several years until I recently acquired a CRKT Hissatsu. The Penatgon is a 5" dagger with one straight edge and one serrated edge and comes with a kydex IWB sheath. It is a well made knife of AUS-6 steel and is very tough. It features a flared, non-slip Kraton handle and is well balanced and fast in the hand. The Penatgon retails for about $105, but can be bought for around $70.
I would not advise the Kobun as a serious carry knife. Cold Steel makes some excellent knives, but as one of their cheaper offerings, the Kobun uses 420 steel which as not as strong as other steels, and doesn't hold an edge very well.
P.S.
If you really want the Kobun, the original version with the blade made of AUS-8A steel is on the closeouts section of their website. AUS-8A is much better than 420 and this is the version one I would buy.