channel estimation based on time-domain pillots
if we ignored the OFDM system for the moment, can we estimate the channel h based on y in my above mentioned system?

Not really, because what you're describing is like if you could sample the channel, but at (symbol rate / 4), if I understood your approach correctly.
Hence, you get aliasing in your channel estimate, if it changes faster than (symbol rate / 4).
Your system, however, sees a channel that has full symbol rate (at least!) in bandwidth.
If your channel is slow enough, i.e. it's coherence time $T_C &gt; 4T_S$, then this subsampling can be OK.
But you haven't given us any info that says it is (if you really ask us to ignore the OFDMness of it all).
If, on the other hand, we don't ignore the fact that this is supposed to be an OFDM system (in which case you really mustn't insert pilots in time domain, that breaks OFDM completely!), then your pilots are far too closely spaced, assuming your OFDM system has more than $N=4$ subcarriers: To use OFDM, your subchannels need to be coherent for one OFDM symbol duration $T_{Sym}=NT_\text{sample}=\frac{N}{f_\text{sample}}$.
I think you might be confusing something. OFDM systems of course use pilots on their subcarriers, which is in frequency domain, that they replace data symbols with every couple of OFDM symbols. But that means the pilots need to be inserted into your data stream, not into your time domain signal.