Hi, Jill; I'm an Audiologist and single mother.
In my years working in a pediatric Speech & Hearing department, I found that kids are motivated to learn and use language when socializing with their peers; so I support the suggestion made by the Speech Therapist. It is great that he will be bilingual, and he can teach some signs to the other kids while he strengthens his skills in spoken English as well as socialization skills with peers.
Some might say you don't need to pressure him to learn English, and I agree with the "don't pressure" part; however, it is so well documented that birth through 5 years are the prime years for language development, he is now 2 1/2 and he has language -- American Sign Language -- but as a hearing child in the United States his education will be in verbal and written English. Therefore, it will be to his benefit if you provide him opportunity to be more exposed to spoken language and to express himself the same way, whether through preschool or playgroups or other activities with his peers.
I've never heard of a preschool that had such a rigid schedule. If you were to choose the 4 hours on 3 days would they somehow penalize you if you started only sending him on two days until he acclimates to the idea? Either way, kids are so resilient he will adapt to the new schedule faster than you will! (unless perhaps he has some other special needs you haven't mentioned).
Don't make the decision based solely on the schedule, either. Go for a couple visits (preferably unscheduled) so you can see how the programs are run. Structure is good, but it is also helpful for him to have unstructured times with the other kids so he can observe their use of spoken language and have the chance to rely on his own abilities to communicate with the other kids at his own pace.
Parenting is the toughest job in the world, and the hardest part is letting go. But with each developmental stage we have to relinquish a little more of our hold on them, and we tend to stress over such decisions. Just go with your gut feeling about which program and its staff and students are best suited to your son's temperament. And if in the unlikely event that your maternal instinct steers you wrong, switch to the other program!
I'll use a personal experience to illustrate my point. My ex-husband and I separated when our son was just about to turn 5, and it was hard on him. When the time came to enroll him in Kindergarten a few months later, I learned that the township would not provide transportation to his Child Care program, so I would also have to enroll him in a Child Care that was in the same school zone as my home. Since he was going through such a difficult transition at home, I couldn't imagine trying to make him adjust to 2 new programs at the same time! So I found out that Kindergarten was not a requirement in New Jersey, and decided to simply keep him in the Child Care he was attending. When he went to 1st grade the following year, he was behind in reading and at first I felt terrible for having let him fall behind. The school put him in a resource room program, and halfway through the year he was already caught-up with the other kids! In hindsight I am glad I did what I felt in my heart was right for my child.
Good Luck to you all!