Is it necessary to replace the bassinet and cot mattresses between use of each child to reduce the risk of SIDS? There will be almost 4 years between my first child (who slept in the cot until he was 3) and new baby. Thank you

A second-hand mattress is suitable to use as long as it remains firm, clean, well-fitting & flat, & has been stored appropriately between use.
Red Nose has the following information.

Sleep baby on a cot mattress that is the right size for the cot, is firm and clean and in good condition with no signs of damage. A soft mattress can increase the risk of SUDI if baby rolls over onto the tummy.
Any previously used mattress should have been well stored, firm and clean. If the mattress is damp, torn or mouldy or there is any doubt about its safety, including fit, it should not be used.
https://rednose.org.au/article/second-hand-mattresses