He has had DSS involved several times for the (now) 4 year old, whom is in his custody. The charges were for neglect and emotional abuse.

If you've never been married to him, he has no legal right to visitation because he hasn't proved paternity nor support. If he makes overtures of coming to visit a restraining order can be procured to keep him away.
If you're divorced, then you must file for sole custody, child support and restricted or supervised visitations. Chances are if he hasn't paid support, he won't show up to court and he'll be responsible for arrearage in support from day one.
You didn't state whether the four year old was yours or someone else's.If you're trying to get custody of the four year old (if he's yours) you'll need DSS to back your request for sole custody.

We live in Massachusetts. I have a 18 month old and a 6 month old by my ex. We were never married. We have not heard from him since November 2009. He denies that my children are his. He has a 4 year old son from a previous relationship & has DSS involved (both my ex and the mother of his first child). He now has a fiance who is 8 months pregnant and also 3 other females pregnant which are said to be his children. I want to fight for sole custody (because the court says that since I was never married to him that I have only sole physical custody). I know he is unfit (obviously). Any man (if there was ANY chance) that the child could be theirs would fight for a DNA test immediately, right? And not just abandon said child? Would that make any impact on a judges decision? Idk what to do.

Good if he denies the kids they are not his that is much better for you in this case you can apply for emergency full phisicial custody you can get it in 10 days good luck

You really do need to file for support before the others do or your children's share is going to be absolutely minisucle if prior obligations are taking shares of his income, leaving a much smaller share available for any subsequent children.