As it says here:
Capacitors tend to be lower precision than resistors. ±20% is not uncommon, and you sometimes even find ones with -20%, +80% tolerance — meaning that if it’s nominally 10 nF, it won’t be less than 8 nF, but it might be as high as 18 nF. Correspondingly, circuits usually aren’t designed to require capacitors to be very precise. If the instructions don’t say otherwise, a 20% capacitor will probably work fine. (In fact if you don’t have the right value cap but you do have one that’s within a factor of 2, there’s a good chance you can substitute it with little or no ill effect.)
The only thing really affected by the value of the capacitor here is how fast or slow an envelope you can get. 4 µF would allow envelopes 4x slower than the 1 µF of the original circuit. If you don’t need such slow envelopes you can continue using 1 µF.
Film capacitors above 1 µF tend to be large and expensive. Look for non-polarized electrolytics. Tayda has 4.7 µF for 12 cents:
(Or it’s possible to use two 10 µF back to back in series:)
