The pension rate for deferral is 1%/9 weeks, NOT 4%.
https://www.gov.uk/deferring-state-pension/what-you-get
I did this calc. for my brother who works in China and he was considering deferral until 70 which would result in a 23% increase in his SP. In his case, taking his pension ASAP would have been financially more advantageous than deferring. Screenshot of calc. attached although yours would be different, of course, but deferral would result in a lower cumulative pension over the years. Although not shown in the screenshot, the crossover point, at which cumulative deferred pension becomes greater than cumulative pension taken at 66, would be 87 y.o. He's decided to take at 66, later this year. Whether he'll then live in a 'frozen' country he's not yet decided but he won't stay in China.