The income from the accepted buyback share is tax-free. There are no short-term or long-term capital gain taxes on the profits from TCS buyback accepted shares. The buyback price of Rs 4500 is the net price you will get for shares accepted.
But the gain/loss from the sell of the remaining share in the open market (not accepted in buyback offer) will be taxable as usual.
Example:
If you buy 1 share for 3700 and TCS accepts it in a buyback offer, the Rs 800 profit will be tax-free.
If you buy 50 shares at 3700 and 25 shares are accepted by TCS for buyback and the remaining are sold in the open market at 4000. Profit from buyback accepted shares (Rs 800*25 = Rs 20,000) is tax-free. Profit from remaining shares (Rs 300*25 = Rs 7500 is taxable.