ITR-1 is a simplified form for individuals with simple income structures, while ITR-4 (SUGAM) is specifically designed for individuals, HUFs, and firms (other than Limited Liability Partnerships).
Most changes in the forms are minor, but since the new, minimal exemptions regime is the default one this time, you will have to specifically opt out of it if you wish to avail of tax benefits under section 80C, 80D, 80G, and so on.
An income-tax notice cannot be issued if three years have passed since the end of the relevant assessment year. However, if there is evidence of concealment of income of at least Rs 50 lakh, a notice can be issued beyond three years but within 10 years of the end of the relevant assessment year.