How long of a future do 4GB cards have on this algo?

Sorry if this has been discussed before but I came up empty on my search. I have a 10gh farm currently of RX3000's and a mix 6600XT's/RX470s and came across a deal on some 4GB RX470's that has me quite intrigued. Just trying to get a judge on how long the 4GB's will be able to mine RVN or if I should just invest in more 8GB units.

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Thanks in advance