The road at junction 41 westbound was closed from 07:00 to 09:00 BST and 16:00 to 18:00 weekdays for a six-month trial, which started last August.
The junction will now be reopened, but a consultation on its future will continue.
Steve Garvey, president of Port Talbot Chamber of Trade, said the trial closure had "decimated" businesses.
He said: "There has been a 20% fall in trade across the board. There has been a huge impact on the footfall since August.
"A couple of stalwarts of the chamber of trade have gone since this came in.
"A ladies accessories business and another T-shirt business have both been lost since August.
"We can't say it was exactly down to the closure, but it will certainly have contributed. Most cafes used to be open until 5pm, but many are closing earlier now.
"The biggest problem we have had is the working day went from 9-5 to 9-3 as the traffic was so congested people left early to avoid the backlog.
"Simply, it meant people were around for two hours less. We are just really relieved it's going to be open again."
Announcing the on-slip road was to reopen, Transport Minister Edwina Hart said: "The final report into the trial closure confirms the primary measure - the overall balance of monetised journey time benefit for the westbound M4 and local roads routes - is Â£180,000 per annum.
"On this basis the trial has demonstrated a benefit to the M4 without overall dis-benefit to the local road network.
"The report also states that car parking, footfall, queue lengths and air quality did not show any significant impact attributable to the trial closure."
Businesses in Port Talbot have seen a 20% fall in trade since the closure of a M4 slip road, it has been claimed.