You can refuse to sign the new IEP. Each year the school must write a new IEP for your child and the one for next year would specify her placement into 3rd or 4th grade. If the IEP says she is to be placed in 3rd grade, you have the legal right to refuse to sign and then they have no legal right to place her in 3rd grade.
Retention can be very hard on a child's self-esteem and on their friendships with other children. Is her speech issue interfering with her academic performance?
Some speech issues can spill over into reading issues - others not. In either case, if you have a plan for what classes or tutoring she will do over the summer, lay out that plan to her teacher. Teachers can and do change their minds when they see parents making good plans for summer tutoring.
What tutoring classes are available at the school? Make sure the IEP should spell out what the school will do for her in terms of tutoring classes next year.
Good luck.

The research is clear, in almost all cases, the long term negative effect of retention outweighs any short term gains.

If your child is struggling, repeating a grade using the same failed teaching approaches is a waste of her time and can do more harm than good.

You say that she is a year behind. In what areas and how is this measured? What is the school doing to help her catch up (beside recommonding retention)?

Often times students with speech and language difficulties also have trouble with reading, writing and following classroom instruction and directions. If this is the case with your child, she needs some specialized instruction beyond speech and language. She may also benefit from the support of a para-professional in the classrrom that can review instructions, help guide her through assignments and re-inforce taught concepts.

Has she been evaluted recently? If not, you may request an evaluation in all areas of concern. This may help you convince the IEP team to add new goals and services to her IEP in areas of need.

Also, if she needs summer services to catch up, the school should provide these as an "extended school year" on her IEP.