Ricin is indeed obtained from the seeds of the castor oil plant:
Castor Oil Plant (Ricinis communis)
This introduced plant commonly grows as a weedy shrub 3m tall in neglected, disturbed areas such as old rubbish tips and filled creek banks. Seeds, if eaten, can cause severe purging, vomiting, abdominal crap, delirium and even death. For man, lethal doses of seed range from two to about eight.
It has uses in 'alternative' medicines as a purgative and in poultices, but the most common uses are industrial (cosmetics, machine oil, varnishes, etc).
Here's some opinions re: inducing pregnancy:
It can be dangerous in some instances to try and interfere with the processes. The two old-fashioned methods of encouraging labour to start (administering an enema or drinking castor oil, both of which stimulate the bowels to produce prostaglandin) are no longer-recommended methods. No matter how desperate you are to start labour, don't try either of these
Castor oil can help to encourage labor in some women at full-term, though it won't work for everyone. Be aware that any method of induction has the potential for serious side effects.
If you do decide to go ahead and use castor oil, I agree that four ounces sounds a bit much. Three teaspoons is about right -- we usually recommend about one ounce of castor oil in six ounces of orange juice.
Use of castor oil in pregnancies at term.
Altern Ther Health Med 2000 Jan;6(1):77-9
Garry D, Figueroa R, Guillaume J, Cucco V.
Winthrop University Hospital in Mineola, NY, USA.
CONTEXT: Despite wide use of castor oil to initiate labor, the obstetric literature contains few references to this botanical laxative. Derived from the castor plant Ricinus communis, castor oil may possess properties that are useful in post-term pregnancies. OBJECTIVE: To evaluate the relationship between the use of castor oil and the onset of labor. DESIGN: Prospective evaluation. SETTING: A community hospital in Brooklyn, NY. PATIENTS: A total of 103 singleton pregnancies with intact membranes at 40 to 42 weeks referred for antepartum testing. Inclusion criteria included cervical examination, Bishop score of 4 or less, and no evidence of regular uterine contractions. INTERVENTION: Patients were alternately assigned to 1 of 2 study groups: a single oral dose of castor oil (60 mL) or no treatment. MAIN OUTCOME MEASURES: Castor oil was considered successful if labor began within 24 hours after dosing. Groups were compared for onset of labor in 24 hours, method of delivery, presence of meconium-stained amniotic fluid, Apgar score, and birth weight. RESULTS: Fifty-two women received castor oil and 48 were assigned no treatment. Following administration of castor oil, 30 of 52 women (57.7%) began active labor compared to 2 of 48 (4.2%) receiving no treatment. When castor oil was successful, 83.3% (25/30) of the women delivered vaginally. CONCLUSIONS: Women who receive castor oil have an increased likelihood of initiation of labor within 24 hours compared to women who receive no treatment. Castor oil use in pregnancy is underreported worldwide. This small series represents the first attempt to evaluate the medication.