This is my understanding of 401k and sep-ira. Take it with a grain of salt, and read the appropriate IRS publications.

You can open an additional 401k or sep-ira for freelancing even if you already have a 401k at work. But the contribution limits are different.

In a 401k, there are 2 types of contributions. Employee deferrals and employer match (or profit sharing). The IRS limit for employee deferrals is $17,500 and all plans are counted in that total. So if you max out employee contributions in one plan, you are not allowed any in the second. The limit for the employer contributions is 25% of compensation (20% if self-employed) up to $52,000 (for 2014), including the employee contributions in that limit. This limit is per plan.

So if you or your husband open a solo 401k, you have to find out how much of your current 401k contributions are employee, and how much is employer from a match. Then, fill up any remaining room in the $17,500 employee limit, followed by 20% of any further earnings. Assuming all of your prior contributions are employee deferrals, your husband could shelter 20% of $33,000, and you could shelter $3000 plus 20% of $1000. If there is some employer match in there, you can shelter more.

A sep-ira is slightly different. There is no difference in contributions, so the only limit is 25% of earnings (20% if self-employed), up to $52,000. But you have to include all other plan contributions in the limit. So your husband would be limited to ($52,000-17,500-5500) or 20% of $33,000, whichever is less. You would be limited to 20% of $4000.

So, TLDR, you are better off with a 401k, and should be able to shelter a minimum of $3200, up to the entire $4000 if at least $1000 of your other 401k was employer match.

Your husband, is better off with a 401k if there was any employer match, and he would be able to shelter $6600 (20% of $33,000) plus about 80% of his employer match. If all of the contributions were employee deferrals, then sep-ira and 401k are the same.

I hope that was clear. :)