[deekumar0203]

There’s two parts to this really:

If the H2 tags are too big, make them smaller by editing the CSS - this is the ‘right’ answer to fix your issue. Add a new class, say, ‘.small-title’, give it a font size that works better for you and apply it to any H2 tags that you want to be smaller. It’s a quick fix that means you can focus on other, more important aspects of your SEO.
That said, it doesn’t make a huge amount of difference, however, SEO is a holistic thing. Google is looking for H2 titles to describe different aspects of the topic you’re talking about - if it sees H4s instead, we don’t know how it perceives this exactly. We know you won’t get penalised or anything nearly so drastic. All we do know is, it’s not absolute best practice (the ‘optimisation’ bit of SEO).