4yr old son has tongue tie, speech delay & gap in lower front teeth. Dentist recommends lingual frenectomy. Will this help?

Maby. Most speech and language specialists will do an oral motor exam as part of their evaluation. If the tip if the child's tongue can easily reach the sites within the mouth needed to form the letters a frenulotomy may not help. If you haven't had a speech evaluation done, i would do so. The therapist can help you decide to proceed with the procedure if they feel it would be helpful.
Yes. Prominent or excessively fibrous lingual frenum can restrict the movement and function of the tongue .It can also cause gaps in the central incisors and gum recession in that area in addition to speech problems.. It will be of great benefit to get it done. When done with laser, it produces very good results.
Likely yes. After 30 years of orthodontic practice, I have seen almost no cases where the lingual frenum causes lower anterior spacing or when resected allows such spacing to close. The attachment is further back. However, for reasons of possible speech pathology or possible periodontal problems, i believe if 2 specialists (speech/periodontics) say yes, then go right ahead.
Tongue tied. Yes, the linguial frenectomy will improve his speech and diction. Typically a labial frenectomy will allow this space to close, however, the lingual (tongue side) frenum may, to some degree, inhibit the closure of the space.
Yes. This question i can answer from personal experience. My son (now 21) also had a lingual frenum that was causing speech problems. I wanted to have it done around age 3 but my wife did not. Finally after much speech therapy in school it became evident that he would need it. It was done at age 7 and it immediately helped his speech.