Carbine length for many years has been 18" so now we are talking about 2" difference, Haz made a good analogy, in close quarters, longer firearms are less friendly, but as Rob pointed out, instructors will take you on, and I think in training, you will find the +'s and -'s of your choice, and the ability to use them even if they are not perfect for the job at hand, any tool is perfect for one situation, but may be applicable to many, understanding the tool and it's capabilities is more important. Just like the cowboy of old who herded longhorn cattle rode 2' wider around the cattle than they would with short horns, learn it and apply it, no harm no foul, it just means different rules of employment. Stand back further from a door, give your self some room on presentation in close quarters, Learn what you have, and implement it, take some courses like Rob's,listen and learn, as you say 4" should not be a deal breaker, just a different mindset to implement it to the best advantage.