Ive just gone Ltd last week.
The letter from companies house was nice...
On 4k a month, after accountant fees you are right, there is little in it either way, however if you are looking to grow it becomes more efficent on higher earnings.
There are other benefits, including paying yourself tax free dividends. Also if your wife was to be a shareholder there are some benefits as well.
A few other points
1. limited liability
2. can look more professional
3.You actually have a seperate entity so you can sell shares or the business much easier if you so wish.
For me personally i just want to build something that is a seperate entity, a proper business if you like that is independant of myself. I also would like to just have a salary and dividends and then know whatever is left in the company can either stay there or be used for growth, i know you can do that with a sole trader but its nice to know that money is company money.
Also a few personal financial reasons were the main thing that swayed me.